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Most people understand that changing tire sizes can affect speedometer accuracy, but not everybody is aware of the effects of tire wear and tire pressure on the speedometer reading.

Tire wear and speedometer error: up to 2% error caused by worn tires!

The original equipment tire for the S2000 is the Bridgestone Potenza S02.  These tires come with 10/32" tread depth when new and are considered worn out when the wear bars are showing, which is at 2/32 tread depth.  That's a difference of 8/32" in tread depth (radius) or 1/4". 

Now 1/4" doesn't sound like much but when you calculate the tire circumference it suddenly becomes significant.  Circumference of a circle is obtained by the formula "2 x pi x r" or multiplying pi by the radius, then doubling it.  So how much does the circumference change when the radius changes by 1/4"?  Well, 2 x pi = 6.28, and 6.28 x 1/4" = 1.57", so the circumference changes by over 1 1/2 inches!

A brand new S02 has a rolling circumference of 75 21/32" or approximately 75.66".  If we divide 1.57" by 75.66" we get .02, which is a 2% difference.  So a worn tire will cause the speedometer to read 2% higher than when the tires were new.  I think that's a pretty significant error, and when coupled with a factory speedometer error of a few percent, it's worth fixing.
 

Tire pressure and speedometer error: up to 1/2% error due to tire pressure!

During development of the Yellow Jacket speedometer calibrator I did some tests of rolling circumference to see how it changed with different tire pressures.  Since I had new tires in both the Bridgestone S02 and Hoosier A3S04 I measured both.  These were mounted on stock 16" wheels.

As you can see, the Bridgestone grew significantly more than the Hoosier did, actually 5 or 6 times as much growth.  I can only assume this is due to differences in tire construction, with the competition tire being much stiffer than the Bridgestone street tire.  While the tire growth of the Hoosier is insignificant, the difference between the 32psi and 40psi measurements for the Bridgestone would amount to about a .5% speedometer error, with the speedometer reading lower at higher pressures.

If I had any information on tire growth versus tire speed I would post it, but so far I've not run across any.  If you have any information on this topic please let me know via the Contact Us button, available on any page at this site.