Do Lower Profile Tires affect Speedometer Reading

Wylie Bean theringmeister at
Wed Nov 9 20:20:27 EST 2005

Ameer hypothesized.....
"I don't think that rule applies to all cases, b/c when the width is
small, changes in the profile doesn't affect the total diameter as
much when the width is larger. That's b/c the profile number is a
percentage of the tire width. Keep in mind the tire diameter is what
affects the speedo reading. The best thing to do is to figure out the
total diameter of the original size and then try to find another size
that is similar to the original diameter. "

........I'm not sure to which rule you're referring, but if it's the
one that both Franco and I put forth, the rule itself absolutely
applies in ALL cases.  It just may be negligible in the case of a
narrow tire.  In any case, variation from the overall rolling
circumference of the stock wheel/tire combo to which the speedometer
is calibrated will have at least SOME affect on the speedometer,
negligible or not.  The (en)tire diameter of the wheel/tire
combination, which in turn directly affects the circumference of that
entity, is what affects your speedometer (and odometer as well for
that matter).  You may also effectively change the gearing of the
vehicle by going to a wheel/tire combo with a measurably different
overall rolling circumference. (ie a smaller combo would create a
quicker car, but would lower terminal velocity.


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