The ORG - 100% FREE advice!

Main Menu

Working out overall diameters of tyres

Started by Skeet, 08 July 2012, 10:39 AM


The formula for working out overall diameters if you want to change rim diameters but keep as close as
possible to original overall diameter is as follows.

Take orig rim diam 14"x25.4=357mm then tyre width 185x80%=148mmx2=296mm+357mm=653mmOD
80 being universal standard profile for a tyre. S0 to inch up to 15" your tyre size would be
15"25.4=381mm then tyre width 205x65%=134mmx2=268mm+381mm=649mmOD the difference being
4mm,acceptable. Your gearing and speedo reading difference will be so slight.

Hope it helps


Fortunately in Australia we have the Tyre and Rim Association who produce an annual publication that has all the common tyre sizes, including the rolling diameter.  But thanks for the formula!  Note that the formula is nominal only and will vary slightly due to pressures, loading and temperature.
1979 6.9 #5541 (Red Bull)
1978 6.9 #4248 (Skye)
1979 6.9 #3686 (Moby Dick)
1978 6.9 #1776 (Dora)
1977 450SEL #7010 white -P
1975 450SEL #8414 gold -P


Goodyear tyres bought one out her in SA in the early nineties of which i have one but nothing since
which is a pity because with all the new sizes coming out. I'm in the tyre game and sometimes wish the
car manufacturers would keep it simple but then it wouldn't make our jobs interesting.