I have a 2.75 gear ratio with a 19 tooth speedometer gear. I changed to a 3.70 gear ratio, what toothed speedometer gear do we need to switch to so the speedometer works right?

1978 Ford F-100 with 351W

 

Transmission? Rear tire size?

 

I believe the Ford drive gears (on the transmission's output shaft) come in 6, 7, 8, and 9 teeth. You need to do a little research to find what you have.


1973-1979 Ford Truck/Van Automatic Transmission Application Chart - FORDification.net

Speedometer Gear Calculator and Charts - TCI® Auto

 

If this is a C4 or C6, the speedometer DRIVE gear is made as part of the output shaft.

 

I am have a c4 for and I think it has a 8 tooth gear. I lucked out and my 4r70w that I am replacing it with also has an 8 tooth gear.

 

I am have a c4 for and I think it has a 8 tooth gear. I lucked out and my 4r70w that I am replacing it with also has an 8 tooth gear.

 

Did you read what I typed in post #4?

The C4 (and C6) speedometer DRIVE gear is made as part of the output shaft, so it cannot be replaced by itself.

The nylon DRIVEN speedometer gear (end of the cable) is replaceable, 16 thru 21 teeth are available, all are available from Ford & autoparts stores.