Which Speedo Gear

I have a 69 with THM 400 427 CI 390 HP. Apparantly this car came with either 273? or 308 gears, according to Chevrolet. The rear gears are now 355's and what I need to know is.......which speedo gear do I need........Please Help.........Thanks.....Steve

[Ken]

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

[akfox1]

I have a 69 with a wide ratio 4 sp, and 3.55 gears. The vendor I purchased my transmission from gave me the red, 21 tooth gear for this ratio. It seems to be accurate, but I've never actually "calibrated" it.

I can only guess if the TH400 uses the same gear, but they both have a 1:1 final drive ratio ( no overdrive).

Good luck.
Aaron

[king80vette]

I'm changing out the 175mph speedo in my car for the stock 85mph one. Does the gear need to be changed?
Steve

[Doug78]

The gears listed and the formula to find the correct gear is for the 4 speed manual. The speedo gears for an automatic are different (more teeth) than the manual. Does anyone know which gear to use for an automatic with 3.55 gears?

[JohnZ]

Originally posted by king80vette
I'm changing out the 175mph speedo in my car for the stock 85mph one. Does the gear need to be changed?
Steve

No - you don't need to change the gear - all speedo heads are calibrated the same; the only time you need to change the speedo gear is for tire size or axle ratio changes.

[LEEJANDZ]

JohnZ -- No - you don't need to change the gear - all speedo heads are calibrated the same; the only time you need to change the speedo gear is for tire size or axle ratio changes

For clarification...
I am switching from a 85MPH to a 175MPH speedometer in a 1979.
I have an automatic.
Do I need to change the gear?

Is the speedometer effectively a plug & play device???



JASON *** BRAND NEW MAGNECOR WIRES FOR SALE!!! *** 1979 L-82

[JohnZ]

Originally posted by LEEJANDZ




For clarification...
I am switching from a 85MPH to a 175MPH speedometer in a 1979.
I have an automatic.
Do I need to change the gear?

Is the speedometer effectively a plug & play device???



JASON *** BRAND NEW MAGNECOR WIRES FOR SALE!!! *** 1979 L-82

Doesn't matter if it's a mechanical cable-driven speedo or an electronic speedo driven by a signal generator at the transmission; all speedo head calibrations respond only to the number of cable revolutions per mile or electronic pulses per mile; changing the 85mph head to a 175mph head makes no difference. Both will indicate the same speed - the 175mph head will just do so with less needle sweep.

[LEEJANDZ]

John -- changing the 85mph head to a 175mph head makes no difference

So all I have to do is switch heads and I will be fine.

Then if I were to switch rear end ratios or wheel diameters, I would have to change the gear... Right?