How far the needle moves is a function of the rotating speed of the speedo cable.

What number on the dial is behind the needle is a function of the dial design. So if you put in a 120 mph speedo, the needle will be at the same place for, say 55mph, as it was on the 85 mph unit (straight up?).

To change the rotating speed of the speedo cable means changing the drive and driven gears at the tranny end of the cable and in the tranny (maybe).

Perhaps another brickster will know whether you can use the 120 mph unit by just changing the speedo cable, and not have to change the internal drive gear.

Good Luck,

Bob

:>)