Remember, the diameter of the tire will affect the number of rotations per mile. For example, if you go from the standard 245/70-16 to 265/70-16 there will be an approx. 3% reduction in the speedo reading and also the odometer reading. So in 300 miles (read by the odometer) you will actually be traveling 309+ miles.

I avg. 16-17 with K&N filter and magnaflow muffler. That's with California gas which has alcohol added. If I go up to Oregon and get a tankful, I'll get 20+ per gallon. Good gas and they pump it!

My driving is combination city/hiway with a 60 mile round trip commute daily. 113k total miles.