Tire Dimension Formula
Taking an example of:
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
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) =
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) =
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.