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The speedo is driven by a gear in the tranny at the tailshaft (I think) which in turn drives the cable.
So to read a reasonably accurate speed there has to be a number of things "in balance".
Rear tire size, which determines revolutions per mile of the rear axles.
Differential gear ratio. "X" tire revs means "Y" drive shaft revs.
Tranny gear, which translates driveshaft revs into cable speed.
Speedo mechanism, which translates cable revs into needle position and also has a gear-driven mechanism to turn the odometer. Trip odo drives main odo.
So changing to the much-desired 120 mph speedo is not impossible, but would require more than just changing the speedo unit.
What I can't tell you is whether or not a 120 mpg speedo tranny gear can be installed and then mesh properly with the 85 mph cable. If yes, then the installation of the 120 mph speedo AND tranny drive gear might be all that's needed.
I had a 1983 245GL with that 85 mph speedo, and did not like it. Rumor has it that the Feds, in their infinite wisdom, determined that drivers like to drive with the needle vertical. So setting it up with the desired (by the feds) 55mph at the vertical point was inportant, more so than having the resulting max reading of 85 mph. Of course, it should be much easier to "Peg it out" if your brick will go over 85 mph!
Good Luck,
Bob
:>)
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