Lift and Tires Mileage difference???

I have a 3inch lift and 33" tires and I was wondering how must is that throwing off the mileage on my truck? Is there a way to calculate the difference so I could find out the exact mileage I'm getting to figure out MPG's??

 

I am running 33's and using GPS i figured my actual speed is about 3mph faster than the odometer reads. A GPS can also help with distance traveled.

 

Yeah with my gps, I found out that at 60mph, I'm actually doing 65mph, but yeah maybe ill look into it with the mileage too

 

You might check out discounttire.com. They have a "tire calculator" on the left that tells you the difference between stock and larger tires.

 

My stock 06 - about 19-20 mpg.
3" with 33s - 12 mpg. Lol