need help on which speedo gear

[rcantu]

hi all,

i just picked up a ttype that had what i thought a broken speedo but it was this gear adjusting box in between the cable and tranny that was bad.

i took it out and now my speedo is showing about 5mph more than actual speed. so if it reads 45mph i'm actually going 40mph.

i was told that it has 3.90 gears and inside the tranny the speedo gear is blue.

what color / # of teeth gear should i get?

thanks for your time.

 

[bison]

You are off by a % not an actual mph. Your off more the faster you go. You need to give us the tire diameter so we can figure out the tire revs per mile. The formula for tire revs per mile is 63360/pi x diameter. 63360 is the number of inches in a mile. Pi x d is the tire circumference in inches. Its easy to go backward from there. We will be able to tell you which gear(s) you need. You may need to change both the drive gear and the driven to get it really accurate but we will wait for your tire height to get the calculations right. Be sure to verify the rear gears too. You would be off more than you stated if the rear gear was 3.9 and the driven gear is blue and the drive gear was stock.

 

[rcantu]

the tire calculator says 27.04 inches.

thanks for your help.

 

[bison]

the tire calculator says 27.04 inches.

thanks for your help.

The closest i could get was to use a 10 tooth drive gear (green) and a 30 tooth driven gear (blue). You already have the blue gear so your good there. Chances are you have an orange drive gear if its the stock one. It has 11 teeth. You can see it through the speedo bullet hole with a small flashlight and a mirror. First you need to locate a green drive gear. Chris at CK should have some. Then just drop the pan and remove the governor cover and assembly and switch out the gear and reinstall. You will be off by 1.37% with that combo. Not too bad. Assuming tire height of 27.04 inches and 3.9 rear gears.

 

[rcantu]

thanks a bunch for your help. i'll probably swap the gear when i get a deep tranny pan to kill 2 birds w 1 stone.