I checked my speed with my stock tires with my gps and my speedo was dead on. Now with the 34's my gps reads 70mph when the speedometer is ready about 66mph.
So, I stayed at 65-66 on my speedometer giving me right at 70mph true ground speed.

The only thing I didn't factor in was the additional miles gained while driving with the bigger tires, Actually your miles are more than the speedo shows, I didn't other doing the calculation but I do know that I had to get gas a LOT sooner than I did with stock wheels.

Big tires effect gas mileage while cruising as well as during take off. I can't buy the idea it doesn't. I can tell it every time I drive mine on the highway.

 

I'm running 12.50X33 Swampers/Aluminum ProComp rims on an '07 Adventure Package.

First tank was 13 mpg.

With 3,000+ miles on the odo now, my last tank came in @ 14.9 mpg (All highway between 70 and 80 mph)

 

on my first tank I got 14.5

(this is with the 33 inch tires and no A/C)

 

well,i changed the oil to synthetic blend,put in new bosch 4 plugs,and with my intake i got 19.6 on the way to nebraska and 19.5 on the way back,that was doing 78 in a 70 and 68 in a 65 all the way there.78mph was halfway point and 68 there on,and the opposite on the way back

 

the first couple of tanks i averaged 15.2 now that I have over 1000 miles I been getting 17.5
maybe it's how I'm driving or break in. I dont know why the increase but I'm glad it happened.