I have heard people talk about fuel millage being in the 20's and maintaining the ideal 1800 +/- rpm, but iI run down the road at 55 mph @ 1500 rpm - 70 mph @2000 in overdrive, 55mph @2000 rpm with overdrive off.In order to keep @ 1800 rpm i have to run 45 mph with overdrive off or 65 mph in overdrive.I am running 305/75/16/10 tires 32.6 in tall with 3.54 gears, would changing to 4.10 gears help. I had factory size when i purchased the truck which was 265/75/16/10 or 31.8 tall and was about the same.I am getting around 17 mpg now but diesel just hit 3.99 gal and gas went down to 3.21 gal Most of my driving is hwy, very little stop and go.I thinking of upgrading to 4 in exhaust (have stock no muffler now) and rv 275 injectorsANY IDEAS ?????

Speed makes a far bigger difference than rpm's do. Run at 60ish for the best economy.

Im not sure with this truck but my 03 got better mpg at 75-80 than 65-70. Im in the same boat as you, I run 305/70/17 (34x12.50) and usually get about 15.5 or so mixed driving. I have 4" exhaust and flow through muffler with bhaf, edge comp and contagious diesel 7x.010 injectors and im still trying to find my best setting on the edge. I recently adjusted my valves but dont know if that will effect mpg any, hopefully it will for the better.

For my 05 65-70 is in the 17-18 range, and 75-80 is in the 15-16 range... that was pre-turbo, I have seen 19 at ~70 with the latest setup.

i have been getting just under 18 on the highway. thats running 65 to 75 most of the time. the truck has 265 75r 16's and we weigh about 7500#. i just installed some rv 275's and i am now getting just over 19. close to 1.5mpg better. i have only run 3 tanks since the install so i am still watching it. the return tee was leaking, fixed that and only one tank has shown the 19.2. so iam still watching it. I would have to agree with AH on the speed thing. that and a heavy foot getting up to speed will kill the milage. mine has stayed very close to 18 highway since it was new.

1,500-1,600 RPM's is the best for MPG's...

Being that diesel is now $4.469 here in New Meadows, ID I really got to watch the MPG's more than speed.

1,500-1,600 RPM's is the best for MPG's...

Do you really think its that rpms, not the associated speed, that effects your mileage? Wind resistance is the #1 factor in fuel economy, which is directly related to speed.

For what ever reason that where the tachometer lands more times out of ten... Yes your right speed is a factor but kicking it down a gear on flat ground back to 2K isn't going to gain much if any at all. I did so some gain but not much (+/- 0.2).This of course is 3.55 gears not 4.10 and a 5 speed trans not a auto... Even like Cummins puts it... You get better MPG staying in top gear as long as possible but keeping your RPM's a low as possible for your load of cargo.Yes towing a TT over a grade you get much better performance at 2K than that at 1.5K...