The computer's wrong...
 

Hi, y'all, but I was sobbing over having to replace my bald tires with tiny original used sized ones, and thought I'd ask a question to forget my sorrow for a moment. As some of you know, I swapped cabs a while back. The donor cab had the overhead computer whilst mine did not. But now, It does! However, my average mpg says around 20 - 22 mpg, while the pump says 13 - 15 (the nearest I can figure, anyway.) The donor had a 5.9, and mine has a 5.2. Is there any simple, easy way to fix this? I recently checked my vacuum, and it hovered around 15 inches of mercury, if this matters or not.... Thanks!! (wait, 265's... sob....)

The overhead lies anyway, you need to do your MPG the old fashioned way.

+1 on that! the lie-o-meter is B/S.

It's going off of PCM data anyway so you need not have it reprogrammed, it's getting the correct info.

Don't for get you put a bigger motor in your truck, and A diff cab as well.

265's? i had 285 70 17 bfg all terrains and had to drop back to 255's because they were 100$ used vs 800 for the ones i had so i feel your pain, being broke sucks.

Well, is there some reason the PCM thinks I'm getting better MPG's? I keep wondering if there's something I forgot to tune, and my fuel economy is suffering for it... Thanks for your sympathy, Sherrif!

All of the MPG readouts on Dodge trucks are inaccurate. 2nd gen, 3rd gen, and 4th gen.
They just give you a very rough estimate.
Hell even the Ford and GM MPG's are off.

If you're hand calculating 13-15mpg then I doubt you forgot to tune anything. That's average for these trucks.