Ring Steel

I just checked my speedometer by using a gps with the car on cruise on the interstate, and I found out that my speedometer is fast. Mine takes 64 mph on the speedo to equal 60 on the highway. I've got the stock tire sizes, in Pirreli P-Zero's; so I am curious about what could cause the speedo error.

Any ideas?
Thanks,
Ring Steel

pistonsuck

You can look at tire specs on www.tirerack.com. It seems like the p-zeros rotate 830/mile and the Bridgestone which would be the stock tire ranges from 837 - 843 / mile. That could have something to do with it. I wonder if they calibrate the speedo with new or half worn tires? More rev/mile mean smaller diameter. Makes sense since your velocity would be directly proportional to you tire radius.

1rev = 2 pi r.
v = dx/dt,
v = # of revs*(2 pi r) / dt

Ring Steel

thanks for the idea.
I checked tire pressure, thinking that lower pressure in the rear tires could cause it; but they were both @ 38 psi.

I guess I'll just have to accept that when my speedo reads 110, I'm only going 102. :-)

Ring Steel