There has been an interesting thread started by Brad Parlee (Re: Spinning at
70mph) concerning calibration of tach and speedo readings after changing tire
size and rear end gears. Different readers have pretty well covered computing
what things should read, calibrating the tach, changings speedo drive gears,
etc. But not much has been said about calibrating the speedometer. Here is a
suggestion on how to do that.
Note the small numbers on the face of your speedometer just below the right
side of the odometer. The number on my Mk IA is 980, but as Ron Fraser has
pointed out, you may see 1000 or 1020. These numbers represent the number of
revoutions your speedometer cable should turn in one mile. Disconnect the
cable from the speedo and let it hang from under the dash. Attach a pointer
to the cable end (I use a paperclip). Measure out a distance of 53ft-9 3/4in
(1/100 of a mile) and mark it on the street with chalk, tape, or whatever.
Using a coat hanger or something similar, hang it out the door of you car so
that it points down at a mark you put on the street. With the door open so
you can observe the pointer, roll the car over the distance between the marks
and count the revolutions of the speedometer cable. For a speedo marked 980,
the cable should make 9.8 revoutions; for a 1020, 10.2, etc. Repeat this
process 2 or 3 times to get a good feel for the accuracy of you efforts.
Depending on the deviation you have from ideal, change the nylon driven gear
on the transmission end of the speedo cable to achieve a revolution count that
is acceptably close to ideal--e.g., 9.6 turns might be as close as you'll get
it. Ron Fraser calculated a 16-tooth gear for Brad's car, which is also what
I figured. But depending on tires, it might take a 17-tooth. A good speedo
shop can completely regear you speedometer head if necessay, but it requires
total R & R of the speedo and, obviously, is going to be somewhat costly.
By the way, I measured a set of old 205/60-13 Phoneix tires and found that the
diameter was 22 3/8in and the circumference was 70 1/2in (a little off
mathematically). With these tires and a 3.07 rear end, I figured an RPM of
3245 at 70mph. Old Dr. Palmer was right again!
Dick Barker
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