Free Formula: Tire Profile/Width Calculator

[DRL]

Tire diameter = wheel diameter (inches) x 25.4 + [tread width x 2 x
aspect ratio]

This will give overall diameter in millimeters; divide by 25.4 to get
inches.

The most useful conversion factor that I, and most likely many others,
derived was to take the tire diameter in mm and divide by 8.5378. This
number will then yield the cars speed per 1000 RPM for a given over all
gear ratio (tranny gear ratio x rear diff ratio).

Example: 255/45 ZR17 tire

diameter = (255 x 2 x .45) + 17 x 25.4 = 661.3 mm (Don't forget your
order of mathematical operations here!)

661.3/8.5378 = 77.46 (I call this the tire-gear number)

77.46/3.45 (over all gearing ratio) = 22.45 MPH/1000 RPM

For me, this is a much more convenient way to compare various tire
options when contemplating a switch.

DRL
NCRS #32566

[Bill Goupil]

Where does the transmission gearing come into play?

[Ethan Pratt (remove camaro to reply)]

I don't want to step on toes, but I'm think the calculation assumes that the
tranny is in final gear (1.00:1), just multiply by the tranny gear to find
RPM in any other gear.

Ethan

[DRL]

The over all gearing ratio is the tranny gear ratio times the rear axle
ratio.
Examples:

2.20 ratio 1st gear x 4.56 ratio rear axle = 10.03 over all ratio

Thus, a car with a drive wheel tire diameter of 681 mm will have a
tire-gear number of 681/8.5378 = 79.76

79.76/10.03 = 7.95 MPH/1000 RPM in 1st gear

Same car, 1.00 ratio 4th gear yields 4.56 over all ratio

79.76/4.56 = 17.49 MPH/1000 RPM in 4th gear

The tire-gear number is irrespective of gear ratio, it applies only to
the tire.

DRL
NCRS #32566

[Joe '91 Z07]

I have one that's easier for those of us who hated math and calculus. Go to
http://www.miata.net/garage/tirecalc.html and they have a JavaScript
calculator that figures what your final speedo reading will be off in %
points. It's easy.

DRL <l88v...@primenet.com> wrote in message
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