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Which Speedo Gear

S

ssvett

Guest
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

Gone but not forgotten
Joined
Jan 30, 2001
Messages
8,236
Location
Hermosa Beach, CA
Corvette
1987 Z51 Silver Coupe
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 :w
 

akfox1

Well-known member
Joined
Sep 8, 2001
Messages
65
Location
Tennessee, USA
Corvette
1969 Lemans Blue Conv. 350, Keisler 5 sp
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

Well-known member
Joined
May 31, 2002
Messages
85
Location
hamburg, pa
Corvette
1980 Blue Coupe
I'm changing out the 175mph speedo in my car for the stock 85mph one. Does the gear need to be changed?
Steve
 
D

Doug78

Guest
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?
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
king80vette said:
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.
:beer
 
L

LEEJANDZ

Guest
;help ;help ;help

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???

;help ;help ;help

JASON *** BRAND NEW MAGNECOR WIRES FOR SALE!!! *** 1979 L-82
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
LEEJANDZ said:
;help ;help ;help



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???

;help ;help ;help

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.
 
L

LEEJANDZ

Guest
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?

;shrug ;shrug ;shrug
 

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