i purchased a 98 f150 2wd that has 02 lightning wheels on it. any idea of how much my speedo/odo is off? Dont know what stock tire size was. But i cant get better than 15mpg and i baby it, shifting @ 1800.

 

Wheel size has no affect. It's the height of the tires. So your question cannot be answered without knowing your current tire size and your stock sizes. You are correct in realizing your odometer is off as well as your speedometer.

 

Stock on a 98 is 16" wheels with most commonly 235/70r16. Lightning wheels are 18" or 20" I believe. You can plug in your info here and it will give you a good estimate.

http://www.miata.net/garage/tirecalc.html