Do Lower Profile Tires affect Speedometer Reading

Ameer Antar antar at
Thu Nov 10 00:49:54 EST 2005

I agree with what you're saying, but I'm not even sure what the rule was. I just looked at what Charles wrote as his list of options. The 185 70 R14 doesn't seem right, so I figured something was wrong. I guess my main point was to just use a calculator so you can be sure of your choices.


   ----Original Message----
   From: "Wylie Bean" <theringmeister at>
   Date: 11/9/05 8:24:04 PM
   Subject: Re: Do Lower Profile Tires affect Speedometer Reading
   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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