trans gear color

[john1977]

Can anyone tell me what gear Iwould need for a TH350 going from a 3:08 rear gear to a 3:55 so as speed reads right?

 

[81 Corvette]

:listening: i've got the same tranny and rear gear and would like to know as well.

 

[Yoda]

Ken did a nice writeup on this some time ago, here is his post

Steve, Richmond Gear has a formula in their Owner's Manual that I got with my trans, and it might help you.

GM Part Numbers (for cable driver speedometer)

• 
  Driven Gear:
• 3987917 - 17 T
• 3987918 - 18T (brown)
• 3987919 - 19T (white)
• 3987920 - 20T (blue)
• 3987921 - 21T (red)
• 3987922 - 22T (black)
• 3860346 - 23T
• 3860347 - 24T
• 3860348 - 25T
  Drive Gear:
• 361002 7T (green)
• 6260705 - 8T (blue)
• 14038093 - 9T (white)
  Driven Gear Holder:
• 345215
  Holder Retainer:
• 3708148
  Bolt:
• 6264903 (1/4-20 x 1/2)

Speedometer Gear Ratio Formula
speedometer gear ratio (SGR) = 63360 X axle ratio divided by 3141.6 X tire diameter

Example:
3.08 gears times 63360 (195148.8) divided by a 26" tire diameter times 3141.6 (81681.6) gives you a SGR of 2.389.

To determine speedometer gears required, multiply the number of teeth on the speedometer drive gear by SGR to determine the number of teeth required for the driven gear.

Example:
SGR X 8 = number of driven gear teeth
2.839 X 8 = 19 teeth on the driven gear.

I don't know if it'll help you, but somebody may find a use for it, eh?

_ken

Bud

 

[john1977]

tried the formula above using the old 3.08 gears with stock 225/70-15 tires that are 27.4" diamiter and stock transmission drive gear (green 7 T ) and driven gear (red 21T) and doesn't work???? This combination of gears and tires my speedometer was right on the money, any ideas??? Maybe i'am not doing the formula correct. Help