On Wed, 26 Feb 1997 Chris Prugh wrote:
"Am I to assume that this number will translate straight across to the
speedo? ie. will a -12.6% difference in circumference create a -12.6%
change in the speedo reading?"
I think so, when you consider that for each rotation of the rear wheels, the
car moves a distance exactly equal to the tire circumference. Since speed is
a function of distance per unit of time, and the speedometer only "knows" the
rpm of the driveshaft, there should be a 1 to 1 correlation. If you cover
12.6% less distance in the same amount of time (same driveshaft RPM), this
equates to 12.6% less speed than indicated on the speedo. Conversely, if you
use a 12.6% larger tire, your actual speed will be 12.6% higher than the
speedo indication.
This same type analysis should also apply to changes to the rear end ratio,
or to final drive ratio changes resulting from the addition of an OD unit.
He also wrote: "...will there be a problem using this profile tire on a
standard 13" x 4.5" wire wheel?
>From what I've read in various tire manufacturer's literature, the ideal
wheel rim width is 70% of the tire's cross section width. Per their
literature, +/- 1/2" does not seem to be a problem. I have no personal
knowledge of this, just passing along some info I have found. For a 185
series tire -
(185/25.4) X 0.70 = 5" therefore, 4 1/2"
would be just within tolerance
Dan Masters,
Alcoa, TN
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