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Calculate final drive ratio?

bsmith809bsmith809 Posts: 1
edited March 2014 in Chevrolet
How can I calculate the final drive ratio of my
Silverado? It originally came with 3:73 gears and
245 75R 16 tires and I want to find out what the
final drive ratio was before I installed the 285
75R 16's Goodyears that I now have on it. A formula
is all I really need. Thanks.


  • rrichfrrichf Posts: 212
    First, the final drive ratio will not change unless the ring and pinion gears are changed in the rear end. What will change by changing tires is the distance covered when the engine turns a specific number of RPM. Tires that are larger will cause the odometer and speedometer to read lower than actual.

    I've never seen the formula that Quad is suggesting. Knowing Quad from this forum I'm sure that the formula is based in fact.

    There is another way to determine the percentage of error by changing tire sizes or brands. Most tire dealers (even Wards) have a book that lists tire sizes and brands and tire revolutions per mile. If it were me I would just look up the revolutions per mile and use the formula of:

    (Old Revs - New Revs) divided by Old Revs

    This will give you the percentage of change from The OLD to the NEW. If the percentage works out to be, say, -10% then when your speedometer is reading 54 MPH you are really doing 60 MPH.

    You also may want to consider having your truck dealer correct the speedometer / odometer for the new tire size.

    Remember that when you increase tire size both acceleration performance and calculated fuel mileage degrade. (The fuel mileage stays the same it's just the calculated number is worse.) However, on the bright side, your commute in terms of miles is suddenly less :)

  • quadrunner500quadrunner500 Posts: 2,728
    >>>First, the final drive ratio will not change
    unless the ring and pinion gears are changed in the rear end. What will change by changing tires is the distance covered when the engine turns a
    specific number of RPM. Tires that are larger will
    cause the odometer and speedometer to read lower
    than actual.<<<
  • mgdvhmanmgdvhman Posts: 4,162
    think of 3 inch tire and a four foot tire...

    roll them both one full turn.....the 4 footer went further...but the center..(driveline) the same no matter what size...

    If the center is turning the same...but the huge tire is covering more ground...(going faster)...then the speedo will read low with bigger tires..

    - Tim
  • robert1972robert1972 Posts: 51
    doesn't that mean that in order for the smaller tire to keep up with the bigger, have to turn a lot faster meaning that the eng. rpm be higher?
  • mgdvhmanmgdvhman Posts: 4,162
    rpm...the bigger the tire...the faster you yes..more RPM's to keep up..

    - Tim
  • robert1972robert1972 Posts: 51
    O.K. so wouldn't that equate to meaning that you have basically changed the gear ratios like going from a 4:10 to a 3:73, one has to turn more rpm (4:10) to do the same speed as the 3:54?
  • mgdvhmanmgdvhman Posts: 4,162
    the gears are still turning the same in the tranny and rear...with the given engine RPM's....the tires are making it "seem" different..

    you could say that has the means of what the tires are doing...but the gearing has not changed..

    Mixed up yet?

    - Tim
  • mgdvhmanmgdvhman Posts: 4,162 a certain degree....make it harder to get going...needing more power..

    - Tim
  • robert1972robert1972 Posts: 51
    OK so the bigger tire makes it harder to get going in, lets say a 5sp with 3:54 as apposed to a 4:10 5sp with the same tires. Or it could also mean if you put the big tires on the 4:10 and the smaller on the 3:54 they might take the same power to move a 6000-7000 ib truck right?
    So it's not what has changed inside, it's basically just what you are feeling by the seat of your pants
    yes I am confused, but I guess it could make sense too.?.?.?
  • bookittybookitty Posts: 1,303
    The axle ratio is fixed and not variable, but the circumference of the tire can be changed, allowing a greater travel distance for one revolution of the axle shaft with a larger tire diameter to a lesser travel distance per axle shaft revolution utilizing a smaller tire diameter. As in all gearing, high gearing means less power and more speed and low gearing produces more power with less speed. It's like the old overdrives. Theoretically you could achieve extremely high speeds in overdrive, except that you ran out of torque and thus limited
    the amount of RPM that the engine could develop. Catch 22 again.

  • moparmadmoparmad Posts: 197
    Easiest way to figure it out,go to and try thier final drive calculator there.I think everyone here is right just a matter of sematics.
    The taller tires don't really technically change your final drive,but in all practicality they do because they do the same thing.I have always heard that changing tire size is the poor man's ring and pinion swap.Well if nothing else hopefully ring and pinion service listed above can help.
  • mgdvhmanmgdvhman Posts: 4,162
    ..maybe a lazy man or easier man..

    Ring and pinion would be cheaper than tires....if you did it yourself?...depending on what tires you got...perhaps even cheaper having it done as well?

    - Tim
  • moparmadmoparmad Posts: 197
    This is true but the average guy can't correctly set up a ring and pinion.Heck my uncle had to take his Ford to three different Ford dealers before he found a so called professional that could do it.
  • bigsnagbigsnag Posts: 394
    If your to the point where your going to have to buy new tires anyway, then you basically get the ring gear swap for free.
  • quadrunner500quadrunner500 Posts: 2,728
    On my Z28, it's easy if you use factory parts. The shims stay the same, and probably won't change. It's when you put in an aftermarket gear that you have to experiment with shims and marking paint, assembling and taking apart to check pattern, by trial and erro.
  • markbuckmarkbuck Posts: 1,021
    Talk # of engine revolutions per mile covered, then everybody will be on the same page. ;o)
  • moparmadmoparmad Posts: 197
    I wish my 8 3/4 was that easy to set.The pinion bearing was bad in the front of it so I had to take it all apart.Otherwise it would have been super easy because the 8 3/4 drops out of the axle in one complete center section(a very heavy one)and my new one would simply slip right in.
    I guess theoretically it is easy to set up but I'm beginning to think there is a little bit of Vodoo involved in getting them absolutely right.
    Well...regardless unless your buying two super expensive tires the tires are probably still cheaper,I don't know about Chevys and Fords but new gears for my Mopar are 155 bucks plus another 100 bucks for the kit to set them up,and another 135 bucks for the tools unless you already have them.
  • quadrunner500quadrunner500 Posts: 2,728
    Mopar, we first tried Richmond gears, but there was more whine than all the Tundra topics combined. Took it apart, put back together 3 times, checked shims, preload, paint patterns. Dang thing was just noisy. Put genuine GM parts in with stock shims...presto! Super quiet. Could just be my experience, but I doubt it. The center section drops out real easy, remove the bearing caps, pull the axle clips. But I agree, not really realistic to be changing axle gears if what you need are tires.
  • mgdvhmanmgdvhman Posts: 4,162
    More whine than all the Tundra topics combined....I like that one..


    - Tim
  • moparmadmoparmad Posts: 197
    Don't even have to worry about axle clips in the Mopar just unbolt the axle flanges and tug.Then the pig drops out the front.First time me and a buddy just grabbed it and pulled it out thinking this thing probably weighs forty pounds.Yeah right...80 pounds of cast iron and laying all kinked up under a 70 Cuda whew what a pain in the fill in the blank.My "new" rearend is a 3.23 suregrip I got well used so now I'm wondering if the gears are just shot.Whines when slowing to a stop no matter how I set it up.But 3.23's are hard to find for the '489 case 8 3/4 and I don't want to go any deeper.I'm building a torque monster 440 to drop and I figure the '.23's will be hurtfull enough and allow me good enough gas mileage to make it from one gas station to the next.
    Besides...if the gears are too deep I won't be able to hear Dwight Yoakam on the stereo.
  • quadrunner500quadrunner500 Posts: 2,728
    3.23 is what I put in the Z. It was perfect. I think you'll like them too. Cool how yours comes apart.

    I think the reason the Richmond Gears whined in my application is because they made a 3.23 to fit a 2 series carrier. With everyone else, you had to go to a 3 series carrier. There must be good reason for this. My problem was solved by putting in an Auburn limited slip 3 series carrier (oem standard), and GM pinion and ring.
  • moparmadmoparmad Posts: 197
    The woahs of being in love with Mopars...they could be renamed Noparts.That is what is available to us from most manufacturers.It is getting better as of late.Many companies are starting to notice us,I guess they are getting tired of our whining.I'll give the 3.23 another shot but right now I'm tired of messing with it so the 2.76 peg leg went back in.To the untrained eye the 2.76 is more impressive 'cause it will burn the right rear about three hundred yards.And the little 318 sounds nasty at 6 grand through the glasspacks...but it really kills your 60 foot times.
    The Z28 sound nice...have fun with her...every good car needs a proper name my Cuda is named Betty the Barracuda,she is part of the family and I've only had to fix the stuff that wasn't broken in the first place.Trust me name your Z28 and it won't let you down.
  • quadrunner500quadrunner500 Posts: 2,728
    Well, I don't have the Z28 anymore. Wife made me sell it so I could have Silverado. I only have one vehicle left that can top 160 mph now, an '86 GSXR1100. Poor me! Life is unfair!
  • andy_jordanandy_jordan Posts: 765
    think of us poor souls that have to be satisfied with 155mph - for us 160 is just an unattainable dream!!!
  • moparmadmoparmad Posts: 197
    When I'm driving the Cuda 65 is plenty fast enough,I just like to get there real quick.Any faster than that and I start to think about those thirty year old spindles,and those thirty year old tie rods,and those thirty year old manual brakes,and thirty years of metal fatigue,and then I back out of her.Oh the hell of being old enough to know better.
  • mgdvhmanmgdvhman Posts: 4,162
    They were 12 years old....and I still had doubts!

    Good old Muscle cars....they did one thing and did it good....went fast in a straight line!


    Who needs to do that?

    Good old 4 wheel drum brakes....

    Every race had two contests....1 to see who could accelerate the fastest.....and the other one to see who could stop before the next intersection!


    I would mash that pedal....the have both hands pushing the steering wheel and both feet on the brake.


    Those were the days..

    - Tim
  • rrichfrrichf Posts: 212
    Ahh yes, back in high school or shortly there after. It seems that the local dealer had an XK150 traded in. The sales man said that he had never been for a ride in a sports car before and encouraged me to take it for a long ride. On the ride, we came down this hill and hit a long straight of about a mile and a half. I nailed it, big time. As the road started to look like a funnel that I couldn't reach into the end, I glanced at the speedometer. It was indicating about 140. The really sad part about this was that the sales man died of a heart attack 3 days later. I've wondered if I started it.

  • mgdvhmanmgdvhman Posts: 4,162
    would make ya wonder??



    - Tim
  • moparmadmoparmad Posts: 197
    My Cuda has state of the art manual disc/drums.That means that you can stop ten feet farther than you wish you would have,instead of twenty feet.But all kidding aside it handles good enough so that by the time that pimply faced delivery boy next to you realizes that he can out handle you,he can't see your taillights anymore.
    Now on a race track it might make a difference,but on the street she is still competitive.
  • gnippergnipper Posts: 120
    I just found this topic and I think the original questions have all been sorted out and a few people have posted calculations used. Just in case though I didn't see a reference to They have several different tire and ratio calculators.

This discussion has been closed.