Drive gear

[Mike Przano]

Can't seem to find the answer in the archives so thought I would ask. Anyone know what tranny drive gear tooth count is needed for a 3:08 rear end. The year is a 61 with a T-10 four speed and 6.70x15 radials. The current gear has 20 teeth but the speedometer seems too slow. Thanks for any help.
Mike

[Terry D.]

Re: Drive gear

Mike
20 tooth is for a 3:36 rear end, and 21 tooth is for a 3:55. I believe it takes a 19 tooth, although I also have an 18 tooth, so I'm not sure which it takes. Hopefully Joe or someone will tell us for sure which one is for a 3:08. According to the Judging manual that ratio was not offered in 61.
Terry

[Verle R.]

Re: Drive gear

What is the mounted diameter of your 6.70x15 tires with running air pressure?

Verle

[Duke W.]

Re: Drive gear

18T would be the proper driven gear (not drive gear, the drive gear is on the mainshaft and connot be changed without disassebling the gearbox) for a '63 with 3.08, and I believe that your '61 would be the same.

What you need to do is obtain some speedo test data. Compare to a modern car, most of which are quite accurate. Take comparative readings at several speeds from 30 to up to 80 MPH.

Compute the PERCENT DIFFERENCE between your car and the reference car. A 20T driven gear with a 3.08 axle should cause your speedo to read low. One tooth less will increase the speedo reading about five percent, two teeth about 10 percent, etc., and vice versa for a greater number of teeth.

You should also verify the axle ratio by turning the driveshaft through whatever number of turns is required to turn the wheels exactly one revolution.

Duke

[Mike Przano]

Re: Drive gear

Verle,
The tire diameter is 86 inches.
Thanks,
Mike

[Al C.]

Re: Drive gear

Sounds like the CIRCUMFERENCE is 86, then the DIAMETER is 27.4 inches
... Al '58 245hp

[Verle R.]

Re: Drive gear

If Al is right and the 86 is circumference and not diameter (BIG tires), the closest gears would be an 8 tooth drive gear and an 18 tooth driven gear.

At 70 MPH the speedomoter will read 70.5, so just a little fast.

9 and 19 would read 75.2

8 and 19 would read 66.8

Verle