Re tire sizes/speedo readings

[Chris Lesher <chris.lesher@cusys.edu>]

Kevin asks:
>I have always wondered in which direction a smaller (or larger) overal
>diameter tire/wheel seup affects a speedometer reading. E.g., does fitting a
>more modern 60 series tire on say, a stock spider wheel from the early 70's
>make the speedo read higher or lower than actual? Why? 

Think of it this way - the speedo measures how fast your mechanicals are
spinning, so it's measuring how fast the wheels are spinning.  For a smaller
diameter wheel to roll the same distance as a larger wheel it has to spin
more times, right?  So, at the same speed a smaller diameter wheel will be
spinning faster.  A smaller diameter wheel/tire combo will make your speedo
read high while a larger diameter wheel/tire combo will have you reading
lower than actual speed.

Chris Lesher
Boulder CO
Looking forward to Alfantazia in June...