Routine general maintenance will also help the mpgs.....

Cleaning the TB...
Cleaning the MAF...
Changing the fuel fuel filter....
Performing these every 15k miles helps....

My question has nothing to do with the vehicles MPG per se. My question has to do with the discrepency between the actual mileage and what the Edge/Gryphon display shows. The actual miles traveled for gallons consumed as calculated by hand/calculator (15.75) are significantly different from the tuner displayed numbers (20.6). It seems the tuner (mine anyway) has a serious calculating error... 15.75 and 20.6 are very far apart.

If you do a lot of gunning the throttle then rolling for long periods of time it will throw off the mpg calculation. 10 seconds of 5 mpg (during a hard acceleration) then 30 seconds of idling down the road at 40 mpg averages out to about 25 mpg. In reality you will get crummy mileage if you are constantly flooring it. Yeah these numbers are just arbitrary but I think that is about what I've seen. I think hills might also have the same effect as the heavy throttle followed by really light throttle.

Other problems would be if your tire size is too big in the programmer and it thinks you are travelling further per wheel rotation.

...That is about all I can come up with at the moment.

I never "gun" the throttle. I don't leave the truck idling. I'm a "fuddy duddy" according to my family when driving. The tire size stock was 255-70/17 and the new tires are only one step larger at 265-70/17. All tire size, differential ratios, etc are set correctly. I just don't understand the large difference in mileage calculations. The mph is within 1-2 mph from the instrument panel and the Edge/Gryphon display. Performance (throttle response in all situations) is improved from stock. Actual mileage seems about the same as stock. It's just that the mpg reading on the Gryphon display is WAY off from reality. I think I'll just switch to one of the other displays so as not to be so annoyed at the unrealistic mpg claim/display.

I agree with that approach. I never found the Avg MPG values to be particularly accurate. But, my truck has a "message center" and it can show MPG too - again, the dashboard values tend to be higher than actual.

I've concluded there's a fault in the algorithm used for the calculation, or, a fault in the ability to sense the amount of fuel used vs distance driven.

I DO monitor Inst MPG though. If the number there is high, I know I'm getting good economy at that moment. At times, I've found by shifting out of OD (when towing) or locking the truck into 2nd gear actually improves my Inst MPG values.

- Jack