Quote:
Originally Posted by billwot
Guys...the speedo reads the rotation of some component in the drive train Different tire circumferences will effect actual speed, and will effect a discrepancy between actual speed and the speed indicated on the speedo, but the tire circumference will not change the speedo reading.
If a 10 inch wheel is turning at 1000 RPM, or if a 20 inch wheel is turning at 1000 RPM, the speedo will still read the same. The only "data" the speedo sees is rate of rotation. You can jack it up and take the wheels off, and the speedo will still read the same.
Tire size alters actual speed. Tire size has no effect on speedometer reading.
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I think that is the point of what people are saying that the wheel size does change the actual speed vs what the speedo is reading so that is the variation.
What seems to be the common belief is that the wheel size has a huge impact on on the difference between the two. You need to have a huge increase/decrease in size of the wheel to really make that big a difference. It takes a 2 or 3 inch increase to show a 5MPH between actual and gauged speed and the little bit of change a worn tire will give is in the fractions of a MPH. All in all not as big a deal that people are making it out to be.