Tire size/Speedo reading

[LEN_LEEB@HP-Sonoma-om2.om.hp.com]

     Kevin wrote:
     Date: Sat, 28 Feb 1998 21:25:19 EST
      I have always wondered in which direction a smaller (or larger) 
     overall diameter tire/wheel setup 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? 
     
     Kevin Murphy
     '74 Spider
     Knowledge for the mechanically challenged.
     
      Kevin,
     think about what happens with a smaller diameter wheel:
     For a given number of RPM's the wheel travels the same numer of revs, 
     but less distance. Since the speedo is actually measuring driveshaft 
     revs, it will read the same for a given engine rpm's. Since the wheel 
     is smaller, the speed indicated will be higher than the actual road 
     speed. 
     For instance, if the speedo read correctly with the old tires, and the 
     new ones have a 5% lower diameter (and hence 5% less circumference) 
     then you will be travelling at 5% LESS speed for a given indication.
     At 60 MPH indicated, the smaller wheels will be travelling at 57 MPH. 
     So you will be travelling SLOWER than indicated. (Speedo reads high)
     Conversely, if you fit larger tires, you'll be going FASTER than 
     indicated. (speedo reads low)
     From a roadspeed point of view, smaller tires = higher indicated speed
     Larger tires = lower indicated speed than actual.
     Len