[GordonGeesey]

Need some assistance figuring out which speedometer gear I need:

My car ('69 GTO) was originally an automatic with the special order 3.90 gear set. The original rear/gear set is still present, but the transmission was switched to an M-20 4-speed manual long ago by a previous owner. I am running original size G70-14 tires on 14x6 rims.

The speedometer reads 50mph while my GPS app says that my actual road speed is 40mph. Close enough to call it a 20% difference, and obviously not close to being accurate.

I have NOT gone into the trans yet to see what color gear I have or to count the teeth.

I found the attached chart on line. It looks pretty simple, but I don't understand what "Drive Gear Teeth" means?? Also, the chart says my correct gear should be green, but the 18-tooth (GM) gear sold by AMES is color-coded as brown. Does the color really matter, or just the tooth count?

Can anyone get me pointed in the right direction to fix this issue?

TIA !!