SpartaPerformance

I changed to a set of 255/45/18 on the rear how can I find the revs/mile so I can recalibrate my speedo?

6-Speed

Here's a tire size calculator that may help. Computes Rev/mile too.

http://www.barretire.com/calculator.htm

gman73

You will also need to multiply that by 1.03 for load.

howarmat

that is the stock equivalent size. i wouldnt change the reading to anything different.

BruceH

+1.

BTW to anyone reading about tire revs. Manufacturers data is often wrong. There is no way for them to know what rim width you are using or what weight will be on the tires. The only reliable method I've found is math. Calculate your own revs when the tire is mounted on your car. Measure the tire height, multiply by 3.142. Take the inches in a mile (63360) and divide by your tire circumference. I've changed tire setups quite a few times and this method is always spot on.

Acerbus 2.0

http://www.1010tires.com/TireSizeCalculator.asp

This one also displays pictures of the tires front and side.

Nuke

I use my GPS to get my real speed and then adjust from there. Since the handhelds typically let you adjust by tens of RPM's (i.e., 710, 720, 730...) you'll rarely get dead-on anyway. Best to have the odometer read a tad high just to keep us... honest.

157dB

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