Well I can't speak from experience, my only diesel truck was a bone stock Powerstroke 7.3, except for a modest lift and 34" tires.

But I agree that EVERYBODY is making outlandish claims on fuel economy gains especially in the gasser segment. This here G-force chip, so many 3rd Gen guys have wasted money on that thing, a stupid IC chip that dynos show NO HP gains and most people LOSE 3-5 MPGs because it throws the O2s off and usually generates a CEL.
Company claims "up to" 60 HP gains and 7 MPG on a Ram 2500. Now talk about a company that the govt. needs to shut down. LOTS of posts by guess trying to send it back under the "30 day guarantee" and getting screwed by the "fine print" too...

 

Under optimal settings you can see a gain from each product, HOWEVER, you've got to be a fool to think you just add the HP and MPG numbers from each product. Air filter gives you 17hp and 2mpg, programmer 60/5, throttle body spacer 10/4, etc. etc. etc.... doesn't mean you'll see 87hp and 11 mpg. Each product may create a gain in a certain RPM range, the range may not overlap with other products, and in fact you're changing the parameters each time you add a product so you may in fact cause a loss.

Anyway, be smart about your modifications, and be realistic. You don't honestly expect to make 500 hp at the tires (600+ at the crank) and 1100 lb-ft and still get 20 mpg, do you? You're doubling the stock power levels, which means 2x the fuel.

 

we have a 02 5.9 cummings w/the power edge programmer and larger injectors. gainned all the power and torque that we wanted and as a side effect we get 26 - 29 mph.

 

If you meant 26-29 mpg, I call bs.

 

Maybe that's what his lie-o-meter told him. :/