> I'd like to check my tach's accuracy. With the NH type automatic
> transmission (1984 Vanagon), what the math to find what I should expect to
>
> read on the tach if I'm doing 60mph?

Well, the 3rd gear ouput on the auto is 1:1, so that makes it a little
easier. The differential in the final drive section is 4.09, so divide RPM
by 4.09 to get the number of times per minute your real wheels are (ideally)
rotating. Then reduce that by a little to account for the stupid non-locking
torque converter---- I'd say a conservative 10% is about right for the far
end. Now, just off the top of my head, "regular" vanagon tires are like 26"
in diameter? So that's about 81 inches around (2 * pi * 13). There are 63360
inches per mile, 60 minutes per hour, so (conveniently)  60mph = 63360
inches/minute. 63360 divided by 81 gives you 782 wheel RPM, which multiplied
times 4.09 for the differential gives you about 3200RPM ideally. Torque
converter slippage will trend that a little higher, how much being
determined by headwinds, hills, air resistance which increases at higher
speeds, etc., but 10% makes a good high end guess, so divide by .9 for 3555
RPM. That means you should be indicating 3200-3500 RPM at 60mph.

So the final formula for ideal MPH->RPM is:

(MPH / 60 * 63360) / (pi * TireDiameter) * 4.09

The reverse, RPM->MPH is:

( RPM / 4.09 ) * (pi * TireDiameter) * 60 / 63360

The torque converter can really monkeywrench any attempt to get an accurate
reading, but the ideal calculation at least gives you a place to start,
particularly if you're considering a tire diameter change. Just copy-paste
the appropriate formula into google's search box, replace
MPH or RPM and TireDiameter with the appropriate values (google will fill in
pi for you if you just type "pi"!) and click the search button.

--
John Bange
'90 Vanagon - "Geldsauger"