I find it easier to rotate my tires myself too (but I don't do the "X" pattern). My tires were the last thing I got at Costco and I haven't been back for 2 years, but even if I had kept my card current, the appointment issue is a hassle. Plus I'd wind up spending $200 bucks in the store while waiting for the shop to finish!
I don't mind rotating tires myself but they get out of round over time and need rebalancing and that's not a DYI job. I hate wasting 2 hours at Discount Tires every 6 months but the tires seem to last longer this way.
I get the free lifetime rotation and balancing. These tires only stores have always been done balancing and rotating in under an hour while I wait over lunch break. I bring my lunch and a headphone radio and they seem to get done real quick.
Have 98 Trooper S bought new. At 50K I got 4 new B'stone 684's. At 99K (this past November) they needed to be replaced. Did a lot of looking and pricing. Settled on 4 Uniroyal Laredo AWP tires 255/70x16. I special ordered them on Sam's Club website, then they notified me when they arrived at the local Sam's store and I had them installled and balanced. The entire package (4 tires, shipping, install, balance, road hazard and free lifetime rotation and rebalance) came to $408 including sales tax. So far they are great - IMO better than the 684's and the size is giving me no problems. I checked tire diameters on virtually every manufacturer's sites and the Uniroyal is marginally bigger at 255/70x16 than some 245/70x16's. Just some input - plus the UTOG tire wear rating is 380 compared to the 684's 180. I have a weekend house on a half mile steep private road in the cold snowy Catskill Myns - private roads are not plowed by the town, so it is ususally 6 - 8" accumulation when I come up. The Laredo AWP's have much better traction in the sonw than the 684's did, plus they stop much better, rarely triggering the ABS vibro-dance as the 684's constantly did even when new.
Actually they were the Select LT equivalent... The local Sam's Club here had their X Radial LT equiv. for $123 each at the same time I paid the $110 each at walmart. I shopped price at the time I bought them and did not find a lesser price AT THAT TIME. The installed price included road hazard, lifetime rotate & balance, mounting, Tenn sales tax @ 9.25%. I keep a Sam's membership, but dropped Costco after their first year here when my free membership ran out and they wanted me to renew @ $35/year. I checked other local dealers in the area and could find lower prices on other brands, but decided to go with the Michelin's instead of the GoodYear Trackers or Liberator's, etc. If you happen to find a great local deal that some tire store has on some 'overstocked' or whatever, jump on them...I have found deals like that in the past and been happy.
Today I checked Sam's and they show $123/each for those... They do have a Mich. XRLT in that size for $102/each, but that is not the LTX M/S (hum...maybe it is the X Radial LT... if so, good price on that, but 8/2002 they were $123). My timing may have been bad since I have seen mention of a $60 discount coupon at Costco lately, but I did not find anything better priced in August 2002 that I wanted to put on the Trooper for the next 60,000 miles.
Someone was wondering how long a tire might sit in the warehouse or shelf before you buy it. A couple of years ago Consumer Repport had an article de-coding the #'s on the tire sidewall - it shows the year and month of manufacture. I will try to fins the story and post the secret.
You can find the tire’s date of manufacture at the end of the Dept. Of Transportation serial number that appears on every tire. The date code is the last three digits. An example code of 228 indicates that a tire was made in the 22nd week of 1998.
This question is directed to those who have experienced the leaking intake manifold gasket: Has anyone experienced the surging/stalling intermittently. Thanks.
I think the first time mine went bad, the Check Engine Light came on, then went off for a while, came on, went off. Drivability was really OK, but eventually the problem progressed so that the CEL stayed on and the physical symptoms (surging idle etc.) were apparent. Eventually the CEL started flashing, and that is usually a more serious problem so I got it into the dealer that day.
Point being, in the early states it could appear intermittently. Are you out of warranty? If not, just take it in. January (cold weather) is prime time for IMG failure by my experience and according to an Isuzu mechanic I am acquainted with.
My 98 Trooper S had this problem at about 55K miles, both the Check Engine light and the surging, but only perceptible at idle. Dealer fixed under warranty (I had extended 100K factory warrantee). Since then everything OK and am at 99.7K now.
Our 1998 Trooper had uneven idle ("surging" sounds too strong for it) and a constant Check Engine Light around 59k miles. Dealer replaced intake manifold gasket under powertrain warranty. That was about 18 months ago. Truck now has 76k+ miles and no additional problems.
Just out of curiosity, what part of the country do you live(warm/cold climate). I just returned from the dealership. The Tech said the mass air flow value was below the acceptable range again, they replaced it last week. They are going to replace it along with the computer. He said he has seen the same problem in several troopers and rodeos'. He also mentioned they have determined the oil consumption problem is due to the PCV valve(if I am remebering correctly, it was early and I hadn't had my coffee, which ever plugs into the right side valve cover(facing the engine). He pulled the hose of upstream of the valve and there was a definite oil residue. Just thought I'd share that. BTW, he did spray carb cleaner around the IMG gasket and didn't find any leak,ie: no surge.
We're in SE Michigan. It's a cold climate, but not arctice like northern Minnesota or the U.P. of Michigan. I'll have to look at the paperwork to see when the IMG on our 98 Trooper failed. I don't think it was during winter, but I'm not sure.
Please jog your memory, or the technician's, regarding the cause of the oil consumption! I changed the PCV valve on our Trooper a couple months ago and noted no reduction in the oil consumption rate. Yes, the PCV valve on the 3.5L Troopers does plug into the engine on the right side, underneath the black plastic engine cover. I am very hesitant to use thicker oil (almost always use 5W30, occasionally 10W30) to reduce the consumption, but will probably try it again if things don't improve. I was in the middle of a promising 15W40 experiment when an unexpected dealer visit resulted in that oil being changed out, before I could really tell if the 15W40 was helping to reduce consumption.
Alrighty ya'll...it's been a few months since I've posted anything and T3 has been doing fine since being rebuilt like the Six Million Dollar Man/Truck several months ago (okay, not $6M, but it was over $10k)...anyway, for some unknown reason tonight my serpentine belt slipped off...pulleys are all there & spinning freely...belt is not worn, no tears...none of the pulleys/etc. seem like they're out of place, but no matter how many times I put it back on, as soon as I start back up the truck, it pops off (after at most 60 seconds)...and it's not like it wasn't working b/c I'm 100 miles away from home & T3 was how I got here...any suggestions that I can try before I have him towed into a shop in the morning?...All help appreciated. -Cobbo
Is the tensioner spring shot (i.e. it is too easy to reinstall the belt)? I never had such a problem myself but it seems like the belt would come right off if the tensioner spring is shot.
I replaced my rear tires with the same size and make like the front (Michelins from Sam's). The front tires had about 300 miles on them, but whenever I had the truck in TOD Auto, I lost traction. I expected this problem to go away with new tires all around, but it didn't. The truck has more traction in rear wheel drive only mode.
Sam - can you describe the circumstances in which you experienced this behavior? At worst, if the TOD were not working, the truck would behave the same. I can not think of any situation short of glare ice where the truck would behave as you have described.
Paisan has noted that if the tire circumference differs by as little as 1/4 inch, the limited slip differential and/or TOD will behave strangely because the differing rotational rates of the tires make the TOD system believe that slippage is occuring. This condition will cause constant slip in the rear differential clutches and will also confuse the TOD system.
What do you mean lost traction? Are you driving in snow or are you going by the TOD display? Same size fronts and backs should not effect TOD or the LSD in the manner paisan has experienced, I believe that was side to side.
The truck has the same exact tires front and back. The front is a few hundred miles older than the front.
Yes, there is lots of snow and some ice in Minnesota now, and what happens is fish tailing when turning, sloping, or accelerating. Also some loss of directional control from the front.
No TOD or other warning lights. And like I said, I have better control in 2WD than TOD.
Just because the tires are labelled the same size does not mean they are. I suggest taking a measurement of the tires to compare. .. Example the Michelin p225/70R16s on my wife's car are nearly the same size as P235s. .. Maybe one pair of your tires is a different model like what is store specific vs general or maybe you have some that were made for original equipment and not the same as the normal retail variety. Read all numbers on the tires. .. Especially there will be a serial number that is required to be there that you can take to a tire store and get the story on your specific tires.
Both front and rear tires were bought from Sam's only 6 weeks apart. Although they are the same everything, I'll double check tire pressure and diameter tomorrow. BTW, they inflated the tires to 35ib, should I reduce it for more traction?
At least some of that is due to the limited slip diff in the rear, which will break both tires loose before the TOD can react and send power forward. That is one reason in serious snow I wish there was a 50/50 4hi option on the Trooper. The possibility of rewiring the TOD controls to do this has been discussed, but AFAIK nobody has ever done it.
Ostaz - I suggest that you have the system checked. I can definitely see why it would fishtail in 2wd, but not nearly as much with TOD engaged. I agree that the limited slip diff would cause fishtailing in 2wd, but the engagement delay is so small with TOD engaged that it would be almost nonexistent. I don't think the SOTF is engaging properly. Thus, your front axles are not getting power. It is only a theory now, but the handling you describe should not happen nearly as much with TOD on.
To test the TOD, engage the system and drive about 10 yards to ensure that everything engages. Then jack up the entire front end and turn one of the wheels. You should move the front drive shaft when you do this. If you leave one front wheel on the ground, you should not be able to turn the jacked up wheel at all.
All other things being equal, your traction would be reduced with higher inflation pressures. You could try dropping them a few pounds.
I checked tire pressure and it was 40/42 PSI front and back (the tire tech told me they did 35, but you know how that goes!). Traction is better now (33 PSI)but still something isn't quite right. My wife's FWD grocery van has better traction in snow and ice.
I will be taking it to the dealer to check the TOD. Can't really do much work myself with a few inches of snow on the ground.
Serrano: You can't do the test you suggest because the SOTF is vacum/electronically controlled, as soon as you turn off the vehicle it disengages.
Slipping: Make sure the diameters are very very close. Any variation will cause problems. What are the indicator lights showing on your TOD when you are having "loos of traction" situations? Is it showing 1 bar? 2 bars? 3 bars? If it doesn't show any bars then the TOD isn't engaging properly and you likely have bad speed sensors.
Pulled a nail in my tire on saturday upstate. Had to throw on my spare tire which is one of the stockers. Of course in my infinite wisdom I didn't disengage the TOD for the first 50 feet. What a mistake! You will KNOW when the TOD is not working properly due to about a 1" difference in tire size. Once I turned off the TOD it was fine. I had to put it in the rear so I reduced the pressure in the other rear tire so that the diameters were similar. Drove about 20 miles to get the tire fixed and all is well now.
Did some TOD driving in snow over the weekend. Had a blast. Tested out various scenarios on a rally road near my place in the Adirondacks. RWD, 4-lo, TOD. All did great, except for the RWD. The TOD I liked cause I could transition from dry pavement to the snowy stuff with ease. I also did a run in 4-lo and had no problem doing 45-50mph with it locked in at 50-50 torque split.
At the end of the weekend I checked my tire pressures which I hadn't checked since Sept. They were at 28psi. I'm guessing that this had a significant impact on how well it handled in the snow. For the trip back on the highway I cranked them to 38psi
hmm interesting. So it's not gripping with the front then. Could be a clutch pack problem or a problem with the SOTF unit, not a problem with the sensors though.
I find it very easy to fishtail with TOD. Even just by flooring it, the back end gets power first and then the fronts engage, causing the back end to pass the front. I find that some middle ground on giving it gas gets me ideal traction, much better than 2WD. However TOD is not perfect, since it is a reactionary system, the backs have to slip first before more than 15% is applied, and when it is supposed to be 50/50, it seems like the rear driveline delivers more "real power" than the front can to the wheels.
I do think it is great in the snow, I have not had any issues going through up to 16" with TOD engaged.
Well SBcooke, you are right, the rear will always feel like it has more power, why you ask? Cause it has an LSD. The front only gets power to 1 wheel or the other, not both.
Maybe it's just my driving style in the snow that allows me to throttle properly with the TOD, but for me the TOD does fine. And when it does cause slippage, I simply engage the 4-low and that is more than adequet. If the conditions are so bad that I need the 50-50 lock torque split I'm not going to be going faster than 45-50mph.
I don't think that lowering the air pressure will compensate for different tire sizes. The overall circumference of the tire is essentially the same regardless of tire pressure. Even completely flat the tire circumference is pretty much the same. Reducing the tire pressure may lower the vehicle slightly, giving the impression of a reduced diameter, but it is only flat on the bottom. The overall distance around the tread doesn't change.
For those that have a different size spare tire, this system should not be counted on to save your LSD. I'm not an expert on LSD, so I'm not sure how forgiving they are to different size tires. As some have noted on this board, it is not a good thing to run even marginally different size tires on the rear. I'm glad, however, that yours survived okay.
I agree, if I needed to travel more than 20-25 miles I would have put the donut spare in the front to save the LSD. It was a relatively short distance at low speeds
You might be right about the tire pressure not effecting the diameter, but I'm pretty sure it does.
gprpodick was (in my view correct) - tire air pressure does not affect the tire circumference. I would add that this is so with regard to the newer (to me - I'm 61) radial tires. In the old days we had non-belted bias tires and I believe the whole carcus of the tire did deflect when flat or severely deflated, and they may well have had a different rolling circumference at that point compared to a fully inflated tire. Then they came out with bias-belted tires that like radials have a continuous basically non-strecthing belt around the tire under the tread to prevent squirming and changes in rolling circumference. Anyway that is my 2 cents.
Aloha again ya'll....just letting you know that it turned out that my serpentine belt problem appears to have resulted from an improper installation of the fan clutch when the front end was redone in Sep. 02...some 8k mi ago...so the clutch sheared off a bolt in the oil pump upon cracking and was causing enough of a bounce on the fan to prevent adequate tension on the belt...long story short it's a follow up to the September work & I wont have T3 back until Wednesday at the earliest...I fear I may have to give up my baby before I ever hit the 2 yr mark at this rate....hope all is well -Cobbo
Tire pressure definitely affects circumference. If it didn't, overinflation wouldn't cause the center of the tire to wear more than the edges. I'm sure it does not change drastically, but it does change.
Below 7 mph, the TOD clutch is disengaged unless it detects slip. Above 7mph, the TOD preloads the clutch pack with enough pressure to transfer some torque to the front end. The more slippage it detects, the harder the clutch pack clamps.
You'll note that if you gas the engine from a dead stop, TOD will automatically engage based on throttle position, not slippage. It is programmed to preemptively engage under certain conditions.
Just browsed paperwork. In Sept 2001, paid $436 + Mich. sales tax ($459.52 total) for 4 Michelin LTX M/S P245/70R16 at Discount Tire. This includes free lifetime rotation and balancing and a prorated replacement warranty.
Ok...that makes more sense. Base price of the tires you paid was about $109 each. Walmart was $110 each. Basic installation which includes lifetime rotation & balance, road hazard, new valve stems was about $9.70/tire. Plus we have about $1.50/tire recycle fee. Then you add 9.25% sales tax. So it looks as if Discount Tire included installation, balance, replacement warranty for no cost. Pretty good deal.
BTW, I just looked at the Discount Tire store locator and there is not one within 300 miles of Memphis.
Actually, the receipt shows they priced the tires at $93.50 apiece and charged a fairly standard amount for the 'extras' (valve stems, mounting, disposal of old tires, etc etc etc). Nowadays, Discount Tire seems to include everything and quote an all-inclusive price, instead of quoting a lower tire price and then adding on the extras.
By the way, Discount Tire has an online store, Discount Tire Direct, that seems to be a legitimate competitor to The Tire Rack. Start at www.tires.com and navigate to the online store instead of the physical store locator.
wlbrown, your $530 price isn't that bad when you consider it includes 9.25% sales tax. That works out to about $485 pre-tax, vs. the $436 that I paid-- only about $50 difference.
Yep...we get hit with 9.25% sales tax and some of our other state fees have gone up the past couple of years. That is the price we pay in Tenn. for having a 0% income tax :-). I think I'm still ahead paying those rather than paying a 4-6% state income tax.
Lowering the tire pressure may not change the actual curcumference of the tire, but it certainly can have the effect desired by the original poster. If you lower the pressure enough to reduce the distance between the center of the wheel and the ground, then you have reduced the effective rotational diameter and increased the revolutions-per-mile. In effect, you can lower the pressure of the larger tire and raise the pressure of the smaller tire to help reduce the difference in rotational diameter.
Having said that, I don't know how much difference this would make in practice, but it can't hurt as a temporary aid as long as all tire pressures are within a safe range.
I expect to keep on rallying if I blow a tire offroad! I suppose the tire difference won't cause the same problem if there is slippage due to off pavement traction.
It seems to me. When you lower the air pressure you don't really change the diameter, in the sense that some have talked about. Changing diameter assumes you are dealing with a circle, in which case we should probably be talking about radius. Reducing the distance from the center of the wheel to the ground becomes a mute point, as you are no longer working with a circle. The distance to any other point on the tire hasn't change, only on the bottom.
Although, the distance from the center of the wheel to the ground has changed, the contact patch of the tread on the ground has increased/lengthened. The shorter the distance from center to the ground, caused by deflation, the longer the contact patch. Basically, no matter how flat the tire, the entire length of the tread still has to pass beneath the wheel and in so doing the wheel will travel a distance equal to the length of the tread.
If you peel the tread off a 245 and a 265 and lay it on the ground, the tread of the 265 is going to be a lot longer. For every rotation of the wheel, the vehicle will move forward a distance equal to the length of the tread. The tread on these tires is not going to stretch or contract very much, no matter how much air you inflate or deflate it to. I imagine that it will a very small amount, but I don't think it's going to be significant on a truck or passenger car tire, especially a truck tire. Maybe every little bit helps, but there is no way the tread length on a 265, 275,or 285 is going to shrink to that of a 245.
Fuel dragsters use tire that have exceedingly soft side walls and tread that allow them to stretch and increase radius with the increase in centrifugal force caused by high spin rates. This in effect gives them a constantly variable transmission, of sorts. But, these tires bare little resemblance to street tires. Truck tires are very stiff and not likely to deform much.
Remember, a tire mounted on a vehicle is not a circle. It's not by definition, as a circle has a constant radius. A tire on a vehicle has a flat spot, an area at the bottom that has a reduced radius and coincides with an increased length contact patch.
But the radius of a flattened tire is shorter than the radius well inflated tire. The circumference may be the same, but the center of the circle keeps shifting so that the radius above the axle is less than the radius below the axle...
Yes, radius keeps shifting but the entire surface of the tread must move across the road for each revolution of the tire, else something has to slip. .. So it is a better to use the turns per mile number from the tire maker's web site as a constant independent of tire air pressure. .. Since there are steel belts that don't expand much with changes in air pressure, deflating the tire will have very little effect on distance traveled per tire revolution.
Comments
Steve, Host
Today I checked Sam's and they show $123/each for those... They do have a Mich. XRLT in that size for $102/each, but that is not the LTX M/S (hum...maybe it is the X Radial LT... if so, good price on that, but 8/2002 they were $123). My timing may have been bad since I have seen mention of a $60 discount coupon at Costco lately, but I did not find anything better priced in August 2002 that I wanted to put on the Trooper for the next 60,000 miles.
Has anyone experienced the surging/stalling intermittently.
Thanks.
Point being, in the early states it could appear intermittently. Are you out of warranty? If not, just take it in. January (cold weather) is prime time for IMG failure by my experience and according to an Isuzu mechanic I am acquainted with.
BTW, he did spray carb cleaner around the IMG gasket and didn't find any leak,ie: no surge.
Please jog your memory, or the technician's, regarding the cause of the oil consumption! I changed the PCV valve on our Trooper a couple months ago and noted no reduction in the oil consumption rate. Yes, the PCV valve on the 3.5L Troopers does plug into the engine on the right side, underneath the black plastic engine cover. I am very hesitant to use thicker oil (almost always use 5W30, occasionally 10W30) to reduce the consumption, but will probably try it again if things don't improve. I was in the middle of a promising 15W40 experiment when an unexpected dealer visit resulted in that oil being changed out, before I could really tell if the 15W40 was helping to reduce consumption.
99 Trooper S
What gives?
Sam
Paisan has noted that if the tire circumference differs by as little as 1/4 inch, the limited slip differential and/or TOD will behave strangely because the differing rotational rates of the tires make the TOD system believe that slippage is occuring. This condition will cause constant slip in the rear differential clutches and will also confuse the TOD system.
Tom
Yes, there is lots of snow and some ice in Minnesota now, and what happens is fish tailing when turning, sloping, or accelerating. Also some loss of directional control from the front.
No TOD or other warning lights. And like I said, I have better control in 2WD than TOD.
..
Example the Michelin p225/70R16s on my wife's car are nearly the same size as P235s.
..
Maybe one pair of your tires is a different model like what is store specific vs general or maybe you have some that were made for original equipment and not the same as the normal retail variety. Read all numbers on the tires.
..
Especially there will be a serial number that is required to be there that you can take to a tire store and get the story on your specific tires.
Sam
To test the TOD, engage the system and drive about 10 yards to ensure that everything engages. Then jack up the entire front end and turn one of the wheels. You should move the front drive shaft when you do this. If you leave one front wheel on the ground, you should not be able to turn the jacked up wheel at all.
All other things being equal, your traction would be reduced with higher inflation pressures. You could try dropping them a few pounds.
Tom
I will be taking it to the dealer to check the TOD. Can't really do much work myself with a few inches of snow on the ground.
Thanks
Sam
Slipping: Make sure the diameters are very very close. Any variation will cause problems. What are the indicator lights showing on your TOD when you are having "loos of traction" situations? Is it showing 1 bar? 2 bars? 3 bars? If it doesn't show any bars then the TOD isn't engaging properly and you likely have bad speed sensors.
-mike
-mike
At the end of the weekend I checked my tire pressures which I hadn't checked since Sept. They were at 28psi. I'm guessing that this had a significant impact on how well it handled in the snow. For the trip back on the highway I cranked them to 38psi
-mike
-mike
I do think it is great in the snow, I have not had any issues going through up to 16" with TOD engaged.
Maybe it's just my driving style in the snow that allows me to throttle properly with the TOD, but for me the TOD does fine. And when it does cause slippage, I simply engage the 4-low and that is more than adequet. If the conditions are so bad that I need the 50-50 lock torque split I'm not going to be going faster than 45-50mph.
-mike
For those that have a different size spare tire, this system should not be counted on to save your LSD. I'm not an expert on LSD, so I'm not sure how forgiving they are to different size tires. As some have noted on this board, it is not a good thing to run even marginally different size tires on the rear. I'm glad, however, that yours survived okay.
-mike
You might be right about the tire pressure not effecting the diameter, but I'm pretty sure it does.
-mike
Below 7 mph, the TOD clutch is disengaged unless it detects slip. Above 7mph, the TOD preloads the clutch pack with enough pressure to transfer some torque to the front end. The more slippage it detects, the harder the clutch pack clamps.
You'll note that if you gas the engine from a dead stop, TOD will automatically engage based on throttle position, not slippage. It is programmed to preemptively engage under certain conditions.
Tom
BTW, I just looked at the Discount Tire store locator and there is not one within 300 miles of Memphis.
By the way, Discount Tire has an online store, Discount Tire Direct, that seems to be a legitimate competitor to The Tire Rack. Start at www.tires.com and navigate to the online store instead of the physical store locator.
wlbrown, your $530 price isn't that bad when you consider it includes 9.25% sales tax. That works out to about $485 pre-tax, vs. the $436 that I paid-- only about $50 difference.
Having said that, I don't know how much difference this would make in practice, but it can't hurt as a temporary aid as long as all tire pressures are within a safe range.
-mike
It seems to me. When you lower the air pressure you don't really change the diameter, in the sense that some have talked about. Changing diameter assumes you are dealing with a circle, in which case we should probably be talking about radius. Reducing the distance from the center of the wheel to the ground becomes a mute point, as you are no longer working with a circle. The distance to any other point on the tire hasn't change, only on the bottom.
Although, the distance from the center of the wheel to the ground has changed, the contact patch of the tread on the ground has increased/lengthened. The shorter the distance from center to the ground, caused by deflation, the longer the contact patch. Basically, no matter how flat the tire, the entire length of the tread still has to pass beneath the wheel and in so doing the wheel will travel a distance equal to the length of the tread.
If you peel the tread off a 245 and a 265 and lay it on the ground, the tread of the 265 is going to be a lot longer. For every rotation of the wheel, the vehicle will move forward a distance equal to the length of the tread. The tread on these tires is not going to stretch or contract very much, no matter how much air you inflate or deflate it to. I imagine that it will a very small amount, but I don't think it's going to be significant on a truck or passenger car tire, especially a truck tire. Maybe every little bit helps, but there is no way the tread length on a 265, 275,or 285 is going to shrink to that of a 245.
Fuel dragsters use tire that have exceedingly soft side walls and tread that allow them to stretch and increase radius with the increase in centrifugal force caused by high spin rates. This in effect gives them a constantly variable transmission, of sorts. But, these tires bare little resemblance to street tires. Truck tires are very stiff and not likely to deform much.
Remember, a tire mounted on a vehicle is not a circle. It's not by definition, as a circle has a constant radius. A tire on a vehicle has a flat spot, an area at the bottom that has a reduced radius and coincides with an increased length contact patch.
Whew. That's my take on it.
No?
..
So it is a better to use the turns per mile number from the tire maker's web site as a constant independent of tire air pressure.
..
Since there are steel belts that don't expand much with changes in air pressure, deflating the tire will have very little effect on distance traveled per tire revolution.