Thanks for the formula! But I still can't figure it out. Try it yourselves
with the figures I got:
DR=(RPM x TD) / (MPH x 336)
TD=22.653 (175/70, 13" tires)
RPM=4500, KPH=112 (=69.9 Am.MPH, =60.44 Brit.MPH) => DR (Am) = 4.34, DR (Br)
RPM=3000, KPH=80 (=49.7 Am.MPH, 43.17 Br.MPH) => DR (Am) = 4.06
RPM=1000, KPH=28 => DR (Am) = 3.87
It makes you wonder, doesn't it....? Maybe I have a changing DR ;-)
I guess I'll have to try the string method as soon as it stops raining.
Thanks a lot,
from a cold AND wet Helsinki, Finland
** From: Kevin Andrews
** In an article in our newsletter, Pat McMullen wrote about tire sizes and
** speed you are traveling with different sizes.So he had a formula. This
** will not work on automatic tranies.You need 3 of these 4 things.
** 2.TD= tire diameter
** 4.DR= drive ratio(final drive)
** RPM is self explanitory,you have tire size but need to get tire
** 205 65 R15 205 is tread width in millimeters.Convert to inches(205
** .0394=8.077). The 65 represents tire profile, a percentage of the tread
** width.So 65% of 8.077 is 5.25 inches. Now rim size is 15 inches in
** So you add that to the tire profile twice to get tire
** inches). Now you got MPH. Now to figure DR you use this formula.
** DR = RPM x TD divided by MPH x 336 (336 is a constant
** will never change)
** Hope this will help. It was a great article by Pat.
** Kevin Andrews
** 68 TR-250(CD7766L)
** Siler City, NC