Steven :
If your car is pre-CF35000, all stock and no overdrive, you can take the
calibration number on the speedo, and multiply by 2.5 to get rpm at 60 mph.
I believe 1120 is the usual value for TR6 (with the 3.7 diff), so at 2800
rpm you are doing 60 mph. Or, take rpm in hundreds (28 in the previous
example) and mentally multiply by 2 and add 1/10. (Thus, 28*2 = 56, + 5.6
= 62 mph, which is pretty close.)
Otherwise, you will need to collect all the info, and use the following
formulas :
Engine rpm = tire rpm * differential ratio * OD ratio * gearbox ratio
(OD ratio is 1.0 if not in OD or OD not present)
(gearbox ratio in 4th gear is usually 1.0)
tire rpm = 20168 / tire diameter(inches)
Tire diameter is best measured by rolling the car (loaded) and counting the
number of turns, but can be approximated by :
tire diameter(inches) = wheel diameter(inches) + (tire width(mm) * aspect
ratio) / 1270
So, a 185/80-15 tire is roughly 15+(185*80)/1270 = 26.7 inches.
Substituting back, we get (at 60 mph), tire rpm = 755, and engine rpm = 755
* 3.7 * 1.0 * 1.0 = 2790 (which again is pretty close. Your tach isn't
that accurate anyway <g>)
Several people have posted Excel spreadsheets, but I forget where offhand.
Try Tony Rhode's web page on speedo repair :
http://ourworld.compuserve.com/homepages/arhodes/Speedo.html
Randall
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