1. Drive down the road and compare your odometer reading to a marked distance. Best bet is 10 miles on the interstate. If your odometer shows approximately 10 miles travelled then you are done. Other wise you will need to do the rest.
2. Pull the spedo-drive gear off the tranny end of the speedo cable and count the number of teeth.
3. The difference is a straight ratio. Namely a/b=c/d, where:
a=Counted number of teeth on the drive gear
b=Number of teeth that should be on the drive gear
c=distance recorded on odometer
d=measured distance travelled.
For example, if you counted 32 teeth on your speedo drive gear, you travelled a measured distance of 10 miles and your odometer indicated you travelled 11.1 miles then:
32/b=11.1/10
b=32(10/11.1)
b= 28.8 teeth required.
so you would need to replace the drive gear with one with 29 teeth.
Get the gears at dealerships or check yellow pages for speedometer calibration shop, they have them. One tooth change is worth about 3 mph change.
I use the mile markers on a flat stretch of interstate and a stopwatch. Hold the speedo real steady for the mile while a friend gets the stopwatch times between the mile markers.
3600 divided by the seconds = true mph.
Compare that to what you held your speedo on. Repeat a couple more times to get a good average.
One thing to remember is that speedo calibration is set for 1000 revolutions of the cable per mile. If the odometer is close but your speed is not you may need a new speedometer head. Speedo heads are available in junkyards, our classifieds sometimes and on eBay.
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National Parts Depot(a sponsor of this site) has a good explanation of how to determine corrections for speedometer differences and also sells the gears. The writeup is in their mail-order catalog. I don't know if it would be listed on their website.
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