Hi Chet:
The thing to remember is that a speedometer is really measuring the
number of wheel rotations per minute that the car is going not the
linear speed. The engineers that designed it knew the relationship
between the number of rotations per minute and linear speed and built
that relationship into the speedo. However, the exact relationship is
dependent on tire size, so the engineers assumed that the tires were
original. If you have a different tire size then the speedo and
odometer will be wrong.
The error is multiplicative, not additive. So, if you had a tire
that had an outer diameter that was 10% too large, then the speedo
would run 10% too slow. If the tire was 10% too small then your
speedo would be 10% too fast.
The current error in your speedo is also probably multiplicative.
In particular, it can't be simply additive because when the car is not
moving, your speedo is presumably reading 0. So, your error should
look something like:
if your true speed is 30MPH your speedo is measuring 33MPH
if your true speed is 60MPH your speedo is measuring 66MPH
Notice that 33/30 = 66/60 = 1.1. Call the ratio of the speedo reading
to actual speed R, so in the above case R=1.1. If you measure R for
your car and want to correct the speedo by changing the tire size, you
will have to multiply the *OUTER* tire diameter by R.
------------------------------------------------------------------------
Hal Levison
Geophysical, Astrophysical, hal@gort.space.swri.edu
& Planetary Sciences (WWW) http://k2.space.swri.edu/index.html
Southwest Research Institute
1050 Walnut St, Suite 429 (303) 546-0290
Boulder, CO 80302 Fax: (303) 546-9687
|