C-6 transmission

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tlindsey18

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Apr 30, 2010

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Midwest City Oklahoma

May 7, 2010

#1

• May 7, 2010

• #1

Could someone tell me what speedo gear would go in a 1971 C-6 with a 2.72 rearend? The hole is completely empty and I have nothing to work with. Thanks

 

2

2+2GT

10 Year Member

Apr 25, 2009

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Southeastern Pennsylvania

May 7, 2010

#2

• May 7, 2010

• #2

If you are asking what driven gear you'll need, you need to tell us your rear tire size, too.

 

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D.Hearne

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Sep 29, 2000

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south louisiana

May 7, 2010

#3

• May 7, 2010

• #3

Probably a 16 or 17 tooth gear. The tailshaft has 7 teeth.

 

rbohm

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tucson,az

May 7, 2010

#4

• May 7, 2010

• #4

need to know what size tire you are running.

 

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tlindsey18

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Apr 30, 2010

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Midwest City Oklahoma

May 8, 2010

#5

• May 8, 2010

• #5

I have 205/70/14s right now, but I intend on going with 255/50/15s or something near that.

 

rbohm

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tucson,az

May 9, 2010

#6

• May 9, 2010

• #6

a 16 tooth driven gear is indicated for either tire you have indicated. one caveat, if you have a 2.73 gear, which is most likely, you need 17 tooth driven gear

 

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tlindsey18

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Apr 30, 2010

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Midwest City Oklahoma

May 9, 2010

#7

• May 9, 2010

• #7

Thank you. i believe I ordered an 18 just on a whim, before this info..that should put me about 5 mph out correct?

 

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D.Hearne

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May 10, 2010

#8

• May 10, 2010

• #8

If you needed a 16, then it's going to be more than that off. Could be as much as 20 mph.

 

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tlindsey18

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Apr 30, 2010

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Midwest City Oklahoma

May 10, 2010

#9

• May 10, 2010

• #9

Response abouve said (one caveat, if you have a 2.73 gear, which is most likely, you need 17 tooth driven gear) so I assumed I would be going one size larger with an 18...???

 

rbohm

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tucson,az

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#10

• May 10, 2010

• #10

going with an 18 tooth gear is going to throw your speedo off substantially, perhaps 10mph. go with the 17 tooth gear.

 

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D.Hearne

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#11

• May 10, 2010

• #11

tlindsey18 said:

Response abouve said (one caveat, if you have a 2.73 gear, which is most likely, you need 17 tooth driven gear) so I assumed I would be going one size larger with an 18...???

Click to expand...

Never assume anything. The gear needed depends on both the gear ratio of the rearend and the tire diameter used. Both of these combined, determine the gear needed. Shorter tires increase the final drive ratio (numerically) Taller tires decrease it. For instance, the final drive ratio of my 77 Comet is just a hair's breadth from the ratio in my 89 Ranger, even though the Comet has 3.50 gears (with a 26.5 inch tire..295/50/15) and the Ranger has a 3.73 rear (with a 28" tire...275/60/15) The Comet should be at 2653 rpms @ 60(but not allowing for the slippage n the C-4 transmission) where the Ranger is at 2687.

 

rbohm

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#12

• May 10, 2010

• #12

tlindsey18 said:

Response abouve said (one caveat, if you have a 2.73 gear, which is most likely, you need 17 tooth driven gear) so I assumed I would be going one size larger with an 18...???

Click to expand...

the reason i put the caveat in there is because when i ran the numbers you gave me through my spreadsheet, i found that using a 2.72 rear gear calced out to a 16 tooth driven gear where as a 2.73 rear gear calced out to a 17 tooth driven gear. the difference is likely a rounding factor since i round the final number to the nearest whole number. besides i have never heard of a 2.72 rear gear.