Converting '68 120 mph Rallye Speedo to 150 mph

crackedback said
It sounded as though only an overlay was being done to convert it. Apparently not the case.

If they change the magnetic force to calibrate the correct MPH for rotations, then it would be OK.

I was trying to state that it could be done with a decal/overlay. A needle return spring and a counter weight provide some adjustment. No it wont be dead on accurate but then they never were and surely aren't now at 40+ yrs old.