Horrible Fuel Mileage

[randrews45]

Best economy mod to make to a diesel truck is to change out the differential gearing from 4. 10 back to 3. 55. I know you will lose some wheel torque but when fuel is $5/gallon and rising, you start to reconsider whether you need that much torque or not. 3. 55 was the standard gear in 1999 and before and engines then had SUBSTANTIALLY less HP, yet few seemed to miss it. Unless you are pulling 20,000+ trailers, you may not really need a 4. 10 diff.



I had a 99 Ram 2500 Diesel 4x4 with 3. 55 gears, got 24mpg in the city and 28mpg highway, now I have a 2002 same model with a 4. 10, get 14 city & 18 highway, planning to have my gears changed back to 3. 55. My old truck w/3. 55 gears at 2000 RPMs moved at 80MPH! My current truck with 4. 10 gears as 2000 RPMs moves at 60MPH! The diff gear ratio really makes a substantial difference in economy.

 

[randrews45]

I chose to do the free spin kit mainly because the truck spends 98% of its life in 2wheel drive. No need to be spinning extra parts/weight. I did that trip with small stock size tires, I think 265/45. I had two flats within 30 minutes of each other about 5hrs out of KeyWest. After spending the night in front of a Firestone shop I bought 4 new tires. Not exactly what I wanted but didnt have much of a choice. Did the trip with those new 265/45/17 tires. Logged everything based off the overhead trip odometer. I am now running the Toyo MT @35" and they are drastically effecting my mpg. I wanted to run the 285/75/17 size but when you get 4 new tires as a gift you dont say no. I think I will need to find a set of wheels and run the 265s for daily driving. I have not logged any mpg with the 35 yet but am positive they are not helping.

You are making a mistake when depending on your auto computer to compute your fuel economy when riding on 35" tires. The computer does not know that you have moved to a bigger tire, so it can calculate the change. In reality, the largest tire size the Dodge Ram 2500 can be reprogrammed for is 265/75R16, a 30"-31" tire. So when you change out for 35" tires, your auto computer is still calculating for a 30" 265 size tire.

The I-285 expressway around metro Atlanta, GA has mile markers every tenth (1/10) or a mile. When I drive 10 miles I see 9. 2 miles exactly on the trip computer, with 35" tires (315x75R16). So rather than having a 10% loss in economy, I am really getting a 10% increase in economy, so take your reported trip computer economy and add 20%, that is your real economy.

Your trip computer is misleading you because it can not calculate the diameter of your larger tires. All it knows is that it is taking more fuel to turn your wheels the same number of revs per mile, because of the larger tires it does not know about.

 

[AMcCormick]

Yes I agree, the only logged miles I have are with the stock size tires. For this reason I dont have any logged miles with the 35s because of the computer being off and have not bothered to recalculate. Just going by my gut feeling. This is a much heavier tire with more rolling resitance. I am also driving my truck much much more than in the past. On the next fuel stop I'll start logging numbers with the 35s and do the math from there taking the trip computer error in to play.

 

[AMcCormick]

The overhead lately has been reading 15-18 with mixed driving and the 35s

This is just the overhead