Mark,
Thanks for the reply. I now think that my tach may be closer than I
orginally thought. In using the equations that went back and forth,
including my own, I thought I was way off - based on speed from my wife
in our new Volvo along side and what I thought was the rolling
circumference of my tires. But, this weekend I marked my tire and the
ground and drove the car for a few revolutions, measured and divided by
the number of revolutions. The result was about 14 inchs smaller than I
was using! I got 61.25" on stock wheels and profile tires - and was
using 75.43 based upon 12" radius suggested by someone as stock. I
realize that tires will expand somewhat once at speed, but I can't imagine
with today's radials the amount is that great. Using the smaller number
and my wife's speed, my tach seems right on. I'll wait for now until I
can have it checked by a device such as yours.
In the meantime, I will probably focus on the speedometer which reads
about 20 mph too high, but the odometer reads correctly (I think), which I
don't understand since I thought (but probably incorrectly) that the same
"gearing" would be applied to the odometer and the needle.
Jay Laifman
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