Need for speed calc.
 

I bought an 85 Carrera with 215/45 17s front
and 235/45 17s rear with all else stock.
I know my spedo is off,but by how much.
I blew past a cop laser with no pull over
and my speedo read 80mph,this cant be true!

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Your tire and wheel combo is just a bit larger than the common 225/50R16. That will make your speedo read a little slower than actual speed. My calculations show that at a speedo reading of 80 mph, your actual speed is 81.

eq: 2 * (width in mm)*(sidewall ratio)/25.4 + rim diameter in inches = tire diameter.

This is approx. the width of the rim will affect actual tire diameter some.

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Bill Krause
'79 911SC Euro

Stock was 215/60 in the rear, on 15-inch wheels.

Try this:

http://www.miata.net/garage/tirecalc.html

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Jack Olsen
1973 911 T sunroof coupe

Try this calculator:


http://www.thegrid.net/thedrivetrainpage/rpmcalc.htm

Cheers,

Joe Garcia