Otto3500

I know I know, another mileage thread.

2006 5.9 Megacab
4x4
Auto
Level kit
285/75R17
3.73 gears
131,000 miles
4" exhaust with muffler behind the cab

I am not going to whine about poor mileage so much, but I am curious your thoughts. I consistently get about 15.2-15.6 mpg, hand calculated. GPS mileage (not odometer mileage) divided by gallons in the tank. I drive 80 highway miles a day to work and back, and rarely on weekends. Typically getting about 460 miles a tank. I drive very conservatively, and the only time I get on the gas to amount to anything is getting on the interstate which is still slow to be honest.

What I am curious about is that I have made a LOT of changes to the truck in pursuit of better mileage, and NOTHING seems to change the mileage at all. Below is a list of the way things have gone since I got the truck and started really tracking it:
- S&B intake without the block off plate installed, then with the block off plate, Smarty JR latest software
-- In this configuration, I tried tons of combinations of tunes from the SJR. Let them each go through 2-3 tanks of diesel, no change. SW1, 2, and 3, with all different combos of timing and torque, even "stock" under each SW#.
- Removed S&B, installed stock intake, ran through a lot of SJR tunes. No change.
- Stock intake, change from ~74 mph (1900 rpm) on highway to ~70 mph (1800 rpm), no change in mileage.
- Stock intake, removed SJR altogether. No change.

All the changes I have made, I let them run for a few tanks and see what happens, but no matter what, the mileage stays consistent. Which really baffles me as the tunes I know are doing different things with the engine because I can hear it and feel it. But mileage just stays the same.

My next step is to check the valve lash, as it *sounds* a bit noisy, but in all honesty, I don't know enough about the Cummins to say one way or the other.

Any thoughts? Any questions about my setup that you might need to know? I have a hard time believing people are getting 20+ mpg with 35s when I drive like a grandma on 33s in a mostly stock truck and can't break 16 mpg no matter what I do.

3J1&3J2Bill

I found that after a heavy tow in the hills improves my fuel mileage. I bought my 2004.5 new and only got 12 mpg empty for two years! Then I had to tow 12,000 lbs. up a long grade and the engine got hot going up the pass. After that trip my mileage increased to 17mpg empty. Was I happy. Now at 90,000 miles I get between 18 -22 mpg. I try to work my truck at some point during the year to heat the engine up and keep it running good.

Danderson

After dismounting my popup camper my mileage jumps about 3mpg to about 16 combined.
With the 1800# camper it drops to about 13.5 with 35's and the equivalent of 4.10's with early G-56.
I lost a mpg with the 35's from my 285-75-17's.
There's other tuning devices to explore with an '05.

Lary Ellis (Top)

Winter fuel is also a culprit if you live in an area that uses it. I lose between 3 and 4 mpg just driving in the winter because of it.