Originally posted by coachreed
Grant-
It looks to me like your figuring a percentage error with the diameter, right? You need to do an error calculation with the circumference as this is the actual distance traveled per revolution.
You're right. Let's see, isn't the equation pi * D?

So, stock = 3.141592 * 29.57 = 92.9"

My new Pirelli tires = 3.141592 * 30.34 = 95.3"

95.3" / 92.9" * 100 - 100 = -2.52%

So when my odo reads 65 MPH, I'll really be going about 66.6 MPH.

No one else has commented about the fact that having an inaccurate speedometer changes the behavior of your airbag deployment and possibly ABS. If your big tires are 8% off, that's a pretty significant piece of false data to be feeding your life-safety system.