{put on dweeb hat}

Not sure if JHARRIS got a straight answer to his question about how tire diameter affects speedo/odo readings.

C=circumference
d=diameter
pi=3.141598

C=d*pi

All you really need to know is that circumference is directly proportional to the diameter and that your speedometer and odometer sensors are just reading tire rotation related to some point in your tranny. Sooo ... if your tire diameter increases by 10% both your speedometer and odometer will read 10% less than actual (your speedo reads 80mph, you're actually going 88mph and your odo reads you went 300 miles, you actually went 330 miles). I'm running 33x12.50s and TomTom tells me that I'm exactly 10% off on my readings.

{take off dweeb hat}