C2 speedometer driven gear help

[Duke W.]

Re: C2 speedometer driven gear help

Revs at 60 are (tire revs/mile) x axle ratio: 766 x 3.08 = 2359, which is why I think you might have a 3.08, but that assumes the tach is reasonably accurate, so you should test it against an accurate tech tach.

Pull the speedometer adapter out of the transmission, verify the color and tooth count of the driven gear, then look inside the transmission and record the color of the drive gear and if possible, count teeth on the drive gear.

There's really no point in speculating until your current configuration is verified.

Duke

[Joe L.]

Re: C2 speedometer driven gear help

Revs at 60 are (tire revs/mile) x axle ratio: 766 x 3.08 = 2359, which is why I think you might have a 3.08, but that assumes the tach is reasonably accurate, so you should test it against an accurate tech tach.

Pull the speedometer adapter out of the transmission, verify the color and tooth count of the driven gear, then look inside the transmission and record the color of the drive gear and if possible, count teeth on the drive gear.

There's really no point in speculating until your current configuration is verified.

Duke

Duke and Don------


Usually, the wrong DRIVE gear will not mesh properly if an incompatible DRIVEN gear is used. However, both of the common drive gears used for 1966 have the same tooth count so even if a wrong combination did happen to work, it should not cause an indicated speed problem in most cases.

The DRIVE gear originally used for a 1966 Corvette with 3.36:1 rear ratio was a steel gear of GM #3708144 and of 1.85" OD. I expect that's still what's in there now. However, it would still be a good idea to check the DRIVEN gear, but primarily for condition. It's very easy to remove but be careful that the gear doesn't fall out of the fitting and fall into the extension housing.

I'm beginning to think that the problem here is in the speedometer instrument, itself.

[Donald H.]

Re: C2 speedometer driven gear help

Ok, I'll pull the driven gear out and check it. Man I need to buy a lift. Jacking it up and setting under jack stands is a PITA

[Joe L.]

Re: C2 speedometer driven gear help

Ok, I'll pull the driven gear out and check it. Man I need to buy a lift. Jacking it up and setting under jack stands is a PITA

Donald-----


...just don't get tempted to use the jack alone and leave out the jackstands for a "quick" job like this. That can be a "ticket to eternal life".

[Donald H.]

Re: C2 speedometer driven gear help

Donald-----


...just don't get tempted to use the jack alone and leave out the jackstands for a "quick" job like this. That can be a "ticket to eternal life".

Yes, I've gotten more cautious in my old age. I not only put in jack stands but also leave a floor jack under both sides

[Donald H.]

Re: C2 speedometer driven gear help

Revs at 60 are (tire revs/mile) x axle ratio: 766 x 3.08 = 2359, which is why I think you might have a 3.08,

Duke

Duke, were does the tire rev per mile come from? How is it calculated?

Thanks,

Don

[Donald H.]

Re: C2 speedometer driven gear help

Duke, were does the tire rev per mile come from? How is it calculated?

Thanks,

Don

Never mind, I figured it out. Mile divided by Tire circumference equals rev/mile. I measured the tire with a tape ( not the most accurate) and I got 27" so the rev/mile with this would be about 747 versus 766. I try to get a better measurement of the tire diameter

[Duke W.]

Re: C2 speedometer driven gear help

Duke, were does the tire rev per mile come from? How is it calculated?

Thanks,

Don

Same places I've listed in the last 10 years - tire dealers - like The Tire Rack Web site - and manufacturers' Web sites.

Revs/mile cannot be calculated using "rigid body" inflated OD because tires distort under load. They are not like steel railroad wheels.

Manufacturers actually measure them and they must be within a tight industry standard range for each size.

Do an archive search is you need a more detailed explanation.

Duke

[Duke W.]

Re: C2 speedometer driven gear help

Never mind, I figured it out. Mile divided by Tire circumference equals rev/mile. I measured the tire with a tape ( not the most accurate) and I got 27" so the rev/mile with this would be about 747 versus 766. I try to get a better measurement of the tire diameter

No, you are wrong.

See my previous post.

Duke