If you have a 120 MPh speedometer an an '82 then either :
1) This is not a USA bike and came from another country, or
2) This is not the original speedometer.
All the USA-imported R65s in 1982 had the mandated 85 MPH speedometer, so they are not older.
In any case, the key thing to look for is the 4 digit code in the lower 'corner' of the speedometer.
It will identify the speedometers ratio, in a rather cryptic manner. Any other speedometer with that
same set of numbers will work in the same way. The 85 MPH speedometer from an R65 should work as a drop as
if they were designed for the same final drive ratio. An 85 MPH speedometer from a bike with a different final drive ration will run a bit slower.
The transmissions on all those airheads of similar vintage were identical, the only difference being the final drive ratio, so the speedometers simply had to be calibrated or set via internal gearing to work with the different final drive ratios that were offered on the smaller bikes -> bigger bikes.
Just to confuse things a bit more for you, the BMW speedometer gauges on many of the bikes tended to indicate 5% - 7% fast, or higher than actual speed.
The 85 MPH speedometers were generally considered the most accurate, or least error of them all. Some people who have switched speedometers have found that getting one which was intended for the next higher (taller) final drive gearing actually tend to make the speedometer error disappear.
Here is a thread that we had on this topic last year:
http://suraklyn.com/yabb/YaBB.pl?num=1191260975/14#14
