<VV> Tirez and curb heights

Padgett pp2 at 6007.us
Thu Oct 20 21:06:47 EDT 2005

>I have always wondered whether the rolling circumference of a tire is pi
>times the loaded radius, or something larger that the tire companies
>determine by experiment.

Yes. That is the formula for MPH from RPM is RPM*1/2 loaded wheel 
diameter/(168 * overall axle ratio). The 168 f.f. contains PI.

So at 2000 rpm in fourth with a 3.55 and a 24 inch tire (loaded) you get 
40.2 mph (close enough for government work).

However, while the tire diameter decreases with load, tires also grow with 
speed (watch the tires on a dragster) and vary with inflation pressure and 
rim width.

Meanwhile I measured my P185/80R13 BFG all seasons and got almost exactly 
24" diameter. The center of the front hub is 11.4" off the ground and 14.9" 
from the lip for the fender (chrome trim). The lip is 25.8" from the 
ground. So it sounds like 15" is a good ballpark for hub center to lip. 
Suspect it has less than 500 miles on the new springs.

There are also several tables on vehicle heights on page 1966-51 of the 
1966 assembly manual which give both "as shipped" and "curb height" for a 
point on the rocker (not the sill moulding) 27.00" back from the front 
wheel centerline. These vary from (curb) 9.10" for a 500 (10137)  to 8.59 
for a Corsa Conv (10767). With the heavy duty Z17 suspension, subtract 
about 7/8". Mine measured 8.7" which is close to the 8.66" spec for a 10567 
w/o a/c.



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