Tech Tips
Tire Dimension Formula

 Taking an example of: P245/50/16 P = Passenger 245 = Tire Section Width in mm 50 = Section height in percentage of width (In above sidewall is 50% of tread section width) 16 = Rim Diameter in Inches

Next let's cover some metric conversion formulas.
Multiply
Inches x 25.4 for Inches to Millimeters (in to mm).
Multiply
Millimeters x .0394 for Millimeters to Inches (mm to in).

With the above definitions understood we can put this info to practical use. I had an El Camino with 195/75/14 tires. The height of the tire was fine but I wanted more meat on the road and in the fender wells. I also wanted to convert from 14" Steel Rims to 15" Aluminum Rims to increase stability and decrease centrifugal weight.

With the formula:
(Rim Inches x 25.4)+(Tire Width x Section Height x 2)¹
¹This is your sidewall height and you have 2 sidewalls in your total height.

So my 195/75/14 Tire Converted to Millimeters =
355.60(rim inch diameter to mm) + 292.50(195x75% x 2 sidewalls) =
648.10mm Tall

I made a spread sheet on an old DOS program, (which I still use..16 years later), and punched in every normal combo I could think of in the 15" size.

I found that a 235/60/15
381(rim inch diameter to mm) + 282(235 x 60% x 2 sidewalls) =
663.0mm Tall

I was able to convert to a 15" rim, get a wider tire, and increase overall height by only app. 15mm, which is a very small amount for such a drastic change. This would be hardly noticeable in regard to speedometer error, and since your car only sits on 1/2 of the tire at a time it only increased my stance height by 7.5mm.

The B.F. Goodrich P235/60/SR15's were a perfect fit and there was "no comparison" to the previous ride. My fenders were filled perfectly with no interference to suspension or body.

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