Speedo or Trans Question

The 23 tooth will get it close but not exact. The problem is that the drive gear inside the trans is an 8 tooth gear and we need a 7 tooth to make the FMS speeedo gears work. Ford changed the gear inside the trans after 89 or 90 I believe. The 23 tooth gear can be purchased from just about any Mustang speed shop. I tried one and it didn't live very long in my trans. The teeth are super thin and usually break off. I'm thinking of changing the gear inside my trans but not sure of the cost yet.

i got a 23 tooth from John's Mustangs in Houston.

it's still working after about 5k on the motor, however it is off... well, it's perfectly accurate (according to my GPS) up to 55 mph, at this point it's about 5% off, getting slightly worse as the speed goes up.

90 GT with 225/55/16s.