I'm running 35's on my truck with 5:29 gearing. My GPS says I'm traveling about 6-7 MPH faster then my trucks speedometer.

Does this mean I am also traveling farther then my odometer indicates meaning I'm getting better mileage then I think?

As it sits I got 17.4 MPG last tank but if I'm actually traveling farther my fuel mileage is better.

 

Wierd, I'm running the same setup (tire/gear combo) but I'm the other way. Doing 60 on the freeway my spedo reads about 66. Maybee previous owner of your truck changed out the spedo gear out to compensate for 31's w/stock gears? Guess it depends on what the stock transmission/diff gearing was on your truck was vs. my '81 stock setup.

Math time!!!
Assuming your gps read 67 and spedo read 60:
67/60 ≈1.116 (correction factor)
17.4 x 1.116 ≈ 19.4mpg = doing better than you thought!

 

Hmm.. Thanks for the info, thats not bad at all.