CORVETTE SPEEDO GEAR TECH
’63-’67 4-speed and ’63-’67 Powerglide used pressed-on steel DRIVE
gears on the mainshaft:
GM #3708144 (“large” 8-tooth), 1.85” O.D., .610” wide, designed for use with nylon gears from 18 to 22 teeth (silver); these “small” DRIVEN gears are 0.8” O.D. and were used as follows:
3.08 = 18-tooth, #3987918 (brown)
3.36 = 20-tooth, #3987920 (blue)
3.55 = 21-tooth, #3987921 (red)
3.70 = 22-tooth, #3987922 (silver, new)
GM #3708145 (“small” 8-tooth), 1.77” O.D., .531” wide, designed for use with nylon gears from 22 teeth (green)
to 25 teeth (orange); these “large” DRIVEN
gears are 0.87” O.D. and were used as follows:
3.70 = 22-tooth, #3860345 (green)
23-tooth, #3860346 (black)
4.11 = 24-tooth, #3860347 (yellow)
25-tooth, #3860348 (orange)
4.56 = 20-tooth, #3860329 (steel – used with special
transmission with 1.92” diameter 6-tooth DRIVE gear)
’68-’70 Muncie 4-speeds used two different 8-tooth NYLON DRIVE
gears, retained on the mainshaft with a spring steel clip:
1.85” O.D. – GM #3924098 (later, #6261794); same “large” O.D.
as #3708144, used the same range of 18-22-tooth “small”
DRIVEN gears, and was light green in color.
1.77” O.D. – GM #3924097 (later, #6261795); same “small” O.D.
as #3708145, used the same range of 22-25-tooth “large”
DRIVEN gears, and was medium blue in color.
’71-up 4-speeds had a larger mainshaft, and used only one DRIVE
gear; 8-tooth, GM #3978758, the “large” size, 1.85” O.D., and used
the 18-22-tooth range of “small” DRIVEN gears. There was no
range of 22-tooth (green) to 25-tooth DRIVEN gears that would
work with this DRIVE gear for 3.70 and higher numerical axle
ratios, so an adapter gearbox was used for those axles along with
the 18-22-tooth family of DRIVEN gears.
NOTE: The above assumes use of stock tire sizes. To calculate the correct
speedo DRIVEN gear for other tire size/axle ratio combinations, use the
following formula:
1)Tire revs per mile – get from the tire manufacturer, or calculate:
Revs/Mile = 20168/tire diameter in inches.
2)DRIVE gear teeth x axle ratio x revs per mile/1001 = DRIVEN gear
tooth count.
(GM speedos turn 1001 rpm @ 60 mph, or 1001 revs per mile)
(1-tooth driven gear change results in a 5% speedo change)
JPH
9/17/13
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