That's a good suggestion to measure the rolling circumference and then
calculate the diameter. It does indeed come out different. After doing
the mark, roll, mark and measure routine in the driveway I came in with the
raw data, fired up the PDP-11 and wrote a Forth program to perform the
calculations prescribed by Steve. Yesterday I reported the diameter to be
22.64, but today I am reporting that the actual rolling diameter is 21.76.
The difference of .88 inch smaller is enough to make my tach error worse by
about 100 for a total error of 500 rpm too low in the 60 - 70 mph range.
Dave
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