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How to get better fuel economy w/your diesel

trendomtrendom Posts: 2
After talking to hundreds of folks across the country I found that the average diesel owner gets 15-16 mpg.

Also what I have found is the "posted speed limit" has everything to do with mpg.

If someone lives where they hardly drive over 60-65 mph unloaded they will most likely brag that they typically get 19-20 mpg.

Someone who drives 75-80 mph will most likely be getting 14-15.

I have personally taken and compared side by side a "broke-in" 05 Cummins and a 05 Power Stroke.

What I found was at if I maintained a speed of 65 mph I averaged 18 mpg. Once I increased my speed to 75 mpg I averaged 14 mpg. SAME TRUCK... 4 MPG DIFFERENCE.

The Cummins actually dropped less mpg than the Power Stroke with the increased speed and I contribute that to the fact that the Cummins hits it torque band faster at lower rpms.


(I did however talk to a guy the other day that installed a performance module called the Workhorse and says it increased his 05 PS MPG from 15 to 20. He also said towing a boat he still got 17mpg. He said he bought it at


  • Hi,

    I'm new to the group, and have a few questions for everyone. Currently I own a quad cab 2500 4x4 diesel Ram and get an average of 14.2MPG with a highway speed of 72MPH. Can anyone tell me from experience how much better fuel milage is switching to a synthetic motor oil? Also can anyone tell me how much better the fuel milage is when using a cold air intake on hte Cummins engine? These are two options I am considering to try to get my MPG to around 18 without easing my foot on the gas pedal.

  • KCRamKCRam Mt. Arlington NJPosts: 3,516
    The Cummins is more affected by gearing than anything like motor oil viscosity. My 96 Ram 3500 4x4 with stock tires and 3.54 axles nailed 24.3 mpg over 270 miles at an almost steady speed of 57 mph. My 05 3500 4x4 with 3.73s and stock tires has no trouble hitting 20 mpg at 55 mph. However, I will lose about 1.5 mpg for every 10 mph over that. If you have the 4.10 axles, it will be even worse.

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  • tex10tex10 Posts: 27
    some guy was telling me that desiels do better on a bigger tire. but he said after 35" inch tale tires every thing gos to hell gas, axles, transmission.
  • cornfedcornfed Posts: 2
    I have researched the aftermarket gear vendor overdrive/underdrive that basically gives the cummins diesel 6 spd transmission 11 gears.

    I did this after test driving a 2500 Dodge cummins 6spd today and found it very disheartening to be running at 2500 rpm at 75 mph. The Rep at Dave Smith Motors i Idaho told me about the option and my internet research seems to be pretty positive, makes sense if you got 11 gears like a semit truck your engine runs more efficient and thus better mileage. the add on is 2900.00 per the dealer. Any thoughts? anybody running one presently? I do alot of highway driving, rarely tow and want the engine to be relaxed going downt he interstate.
  • mullins87mullins87 Posts: 959
    Gearing is all important with a diesel engine, that's why big trucks have multiple gears. The ones I've had any experience with had 9 speed Eatons, shift 1-5 in low range, then hit high range and go back to 2nd to get 6th gear, then on up to 9th. Unless you can switch the range on that overdrive unit while underway, you'll still only have 6 gears. I admit, I haven't looked at the Gear Vendor unit in a fairly long time, maybe they changed that.

    Do you have a 4.10/4.11 gear ratio? 2,500 rpm sounds about right for 75 mph with that ratio. I had an F-350 with 3.73's in it and it ran around 2,100 rpms at 75 mph. I'm afraid that if you go with an overdrive unit, your highway performance will suffer. Turbocharbed diesels need some rpms to get the turbo spinning fast enough to create boost, which in turn builds HP. Find out the overdrive ratio, is it a 15%, 20%, etc... Reduce your engine rpms by that percentage to figure out what your engine speed will be at a given speed in 6th gear. Then get out on the road and try driving in 6th gear with the engine near that rpm. You'll probably want to do this on a not so busy road since you'll probably be driving slowly. Then roll into the throttle. If the truck has plenty of "get-up-and-go" to it, then the overdrive may be an option. I'm betting the engine won't be spinning fast enough. I know how my F-350 drove at 1,600 rpm in 6th gear, even with a chip.

    Another option is to just regear your axle. I think that would be considerably cheaper.
  • Has anyone installed the Bosch Upgrade injectors? I have a 12 valve cummins and was wondering if they noticed a difference in their mph or torque and horsepower?

    here's a reference link: the exact ones I bought:
    auction from the guy I bought from
  • ezshift5ezshift5 West coastPosts: 853

    ..I couldn't agree more.

    ..doing 55 mph (absorbing big time nasty looks from many citizens), my diesel will turn in that same figure in mpg. Increase to average citizen speed and fuel efficiency heads for the cellar. I plan ahead, drive 55 and try to avoid eye contact.......

    all the best, ez
  • KCRamKCRam Mt. Arlington NJPosts: 3,516
    Same here... at 55mph, I get a solid 20 mpg in my 05 Ram 3500 Quad Cab 4x4 dually (Cummins, automatic, 3.73 axles). At 65, that drops to about 18.

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  • I'm buying '98 2500 Laramie SLT diesel 4X4 with automatic transmission. I'm looking to get better mileage than my 2000 1500 gasser. Keeping track of how efficiently I'm driving is important. The truck I'm buying has an overhead console but doesn't give the average MPG. Does anyone know if this is just something wrong with this particular console, or if direction and temperature are all it every showed. If it's a model that only shows direction/temp or is just not working on the MPG function, does a replacement console have to be specific to a diesel? Also, if the existing console came only showing direction & temperature, would the wiring be in place for a console that will show MPG? Is everyone totally confused now by what the heck I'm talking about? If not, thanks for your help.
  • Here's my mileage numbers on my 2004 Dodge Ram 2500 diesel quad-cab with a 4-speed automatic. We started the journey with approximately 9,500 miles on the clock. We averaged 19.49 MPG.

    Col1 Cumulative Miles Travelled
    Col2 Actual Miles Travelled
    Col3 Number of Gallons
    Col4 Miles/Gallon
    Col5 Price/Gallon
    Col6 Fill-up State
    Col7 Fill-up cost
    Col8 Comments

    Col1 Col2 Col3 Col4 Col5 Col6 Col7 Col8
    389.1 389.1 22.20 17.53 $3.16 CA $70.13 Mostly freeway at 80MPH
    640.9 251.8 14.00 17.99 $2.86 OR $40.01 Montain, Coast and freeway
    1003.2 362.3 17.07 21.23 $2.73 OR $46.59 Mostly freeway at 65MPH
    1352.2 349 17.69 19.73 $2.99 WA $53.04 Mostly freeway at 70MPH
    1702.8 350.6 16.97 20.66 $2.84 WA $48.18 Freeway, mountains, traffic
    2143.6 440.8 22.24 19.82 $2.79 OR $62.02 Mostly freeway at 70MPH
    Average MPG 19.49

    These numbers are based on a round-trip journey based on driving up I5 from CA into Vancouver BC, and then back into CA. This includes moutain passes in northern CA and southern Oregan, as well as driving from Vancouver BC into Whistler. Note that I haven't included the last fill-up here, since I haven't filled up the tank yet!

    Some points of interest... diesel is way much cheaper in OR & WA. In fact I think that us Californian's are getting ripped off. I'm not sure why diesel is so much more expensive in CA than the other states. Any ideas? Would be interested to hear what others are paying in their locale.

    The mileage numbers are based on using the tried and tested method of Miles travelled divided by number of gallons. I haven't checked the onboard computer... but I don't know if I would necessarily trust it.
  • In reply to steedoflondon. I live just outside of Philadelphia. I haven't paid a lot of attention to the cost of diesel because I didn't have a diesel, but would occasionally notice that the cost of it was about 25 or 30 cents more per gallon than 87 octane. I'm in the process of buying a Dodge Ram 2500 diesel so have been paying more attention lately while I've been looking. At most stations that carry diesel the price is still higher than 87 octane, but several weeks ago I was at a station and diesel was 7 cents cheaper, $2.459 vs $2.529 for 87. Today I went by the same station and diesel was still $2.459 but 87 was $2.599, a 14 cent difference.
    Can anyone also recommend a good light truck tire that will help get good mileage? I'm never off road, don't tow anything, and don't usually have a very heavy load. I do, however, need something that will get me through wet roads, snow, and ice, without costing a fortune. I'm trying to save money, not just shift the spending from gas to tires.
  • Just one point to add to my original posting. The onboard trip computer in the truck states that I got 18.7MPG during the trip. I'd actually expected it to be more optimistic than my calculations.

    During the trip I only noticed a couple of places that had diesel less than the middle or premium grade of gasoline. I didn't see a place that was cheaper than regular.

    in response to Farmboy4, have you considered taking a look at Biodiesel (e.g. and then seing if that is cheaper than #2 diesel in your area?
  • bruchkobruchko Posts: 3
    I recently bought a 2000 F-250 Crew Cab Lariat SB with the 7.3 Powerstroke with 68,805 mi on it from a dealer. It's a 1 owner vehicle and is in great condition. When I test drove it the mpg readout said 17.1. I took it home for the weekend and it was down to 15 when I took it back on Monday. Since buying it 2 days later (2 weeks ago) it's now getting 13.3 mpg. The dealer has a good reputation though I've never bought from them before. The fuel filter and air filter have been changed, it runs fine and I'm not driving it hard at all. I've tried getting fuel from different places but no change. Any suggestions?
  • I'll only speculate that the 17.1 readout was the average of what the previous driver had been getting on the way he was driving, maybe mostly highway miles, and what you're seeing reflects a different type of driving, maybe more stop and go.

    Also, if you go to the AAA website at, click on the Automotive tab. In the upper right side of the next window you'll find a spot to click on for fuel prices in your area. Here's a comparison of stations that sell diesel within 10 miles of me in Glenside, Pa.

    Brand Regular Mid Premium Diesel
    Hess 2.62 2.72 2.82 2.56
    Gulf 2.62 2.80
    Hess 2.62 2.82 2.60
    Sunoco 2.64 2.80
    Texaco 2.66 2.76 2.88 2.99
    BP 2.66 2.92
    Hess 2.68 2.74 2.60
    Sunoco 2.70 2.80 2.77
    Mobil 2.73 2.87
    Sunoco 2.76 2.84 2.85
    Sunoco 2.78 2.80
    Exxon 2.90
    Gulf 2.86

    You'll notice that Hess is the one that sells diesel cheaper than regular. Someone told me yesterday that Hess has their own refinerie so doesn't have to buy it from someone else.
  • bruchkobruchko Posts: 3
    True, I am doing more stop-and-go driving. I think the previous owner pulled a horse trailer or an RV. I only pull a pop-up camper or a fishing boat from time to time. This truck is in almost brand-new condition throughout. I had my mechanic give it a once-over and he thought it was a great deal. I paid 23K.

    A local diesel repair shop said my fuel jets are probably dirty though I would think a dealer would clean these out before selling the vehicle. I'll try that before looking into a performance chip. I don't need more horsepower, just better fuel economy. I'm looking into the "Workhorse" performance module as a possible solution if I need to. The dealer says it doesn't inject more fuel, just burns it more efficiently.

    Here in the Parker CO (just SE of Denver) diesel is between 2.56 and 2.69 depending on what part of town you're in.
  • I have a 94 Cummins, just bought it.... I power washed the motor last night and when I was about to turn it on..."water in fuel" light came on....I did leak the filter like the manual says.... but the light keeps coming on.. will this light go off after several driving cycles,,,Please help..

  • Diesel in Walla Walla Washington, SE corner of Washington, is 3.30/gallon. :cry:
  • Ok i just wanted to know about the mph for my new truck. Truck is the 2002 F250 XLT CREW CAB SHORT BED 7.3L diesel, 4x2 Can someone tell me what kind of mileage i should get in the city/ highway. I am not planing on towing or hauling anything.
    Thank you :confuse:
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