How many teeth?
Use a GPS, Garman, Phone to drive at 60 mph then record what the speedo says.
My bet is that there is something else going on here and your 40 = 80 is not accurate enough because that says your speedo gear is about 2x as small as it should be, hard to believe.
Something else that can effect this is the speedo internals, but do the easy stuff first.
If its an Open diff keep one tire on the ground. If Sure Grip both wheels off the ground.
Use a GPS, Garman, Phone to drive at 60 mph then record what the speedo says.
My bet is that there is something else going on here and your 40 = 80 is not accurate enough because that says your speedo gear is about 2x as small as it should be, hard to believe.
Something else that can effect this is the speedo internals, but do the easy stuff first.
