How difficult is it to change the driven speedometer gear in a C-4 trans?
Thanks

 

Pretty simple, follow the cable down to the transmission, loosen the bolt holding the cable assy in the trans and pull it out. There is an O ring on the cable housing and I would suggest you clean around the cable boss area prior to removal so no dirt falls into the trans. There is a horseshoe style clip on the gear itself holding the gear in place. Prior to removing the clip, see how well it is seated so you know what to look for upon reassembly. I use oil, vaseline or brake grease to lube the o ring prior to installation. Remember the trans housing is aluminum so don't smoke / over torque the retaining bolt.

 

Thanks much for the info, sounds pretty easy.

 

Now that I know how to change the gear, is there a way to determine which gear to use?
The speedo is off by about 10mph, rear axle is 3.31, tire size is 7.15x15, with the C-4 hooked up to a 351W.

 

Yo Bro, here you go

http://www.tciauto.com/tc/speedometer-gear-calculator
FORD SPEEDOMETER GEAR INFORMATION



Ford Speedometer Gears

Drive Gears Driven Gears
6 teeth - machined into output shaft (C6 only) 16 teeth - tan
7 teeth - machined into output shaft 19 teeth - tan
8 teeth - machined into output shaft 20 teeth - orange
9 teeth - machined into output shaft 21 teeth - red
23 teeth - white 1



- Ford drive gears are machined into the output shaft on C3/C4/C5/C6 & AOD transmissions. You may need to remove your tailhousing to determine how many drive teeth your transmission has.
1 - This is a retrofit gear to allow the speedometer to function with 3.73 and 4.10 rear end gears. Since the gear tooth pitch is slightly different than OE, the service life of this gear is limited to less than 25,000 miles.

 

Firstgothenshow,
You've been a great help-thanks.