Speedometer gear & odometer

[David K.]

I am currently preparing my '67 for a PV next month. I measured my speedometer using a GPS while driving and checking it at various speeds, it is about 9.2% off (speedometer reading is higher/faster than what the GPS measures). There is no needle bounce at any speed. The odometer is also 9% off showing 11.4 miles on a 10.5 mile course. My car is a 427/435 car with a 3.70:1 rear end (this has not been changed).

I removed the speedo gear and it is a 20 tooth, blue gear and it appears to show some wear (some teeth are rounded and some dip toward the middle slightly). My questions are:

1. Is this the correct gear? If not what is the correct one?
2. Would the apparent wear be enough to case the 9% difference?

Thanks for the help.

[Michael H.]

Re: Speedometer gear & odometer

David,

A July 74 printing of the parts book calls for a 3860345 22 tooth green driven gear. The blue 20 tooth is shown for a 3.36 diff. Looks like the drive gear (inside) is the same for both.

Tire size would have an effect on accurace also. The gears listed are for the original tire dia.

Michael

[Wayne M.]

Re: Speedometer gear & odometer

David -- As Michael says, you should have the green plastic 22-tooth driven gear. The blue 20-tooth one is of a smaller diameter, and is wearing and eventually will skip/slip on the steel 8-tooth drive gear (1.75"od). Driven gears below 22-teeth require the larger drive gear (1.85" od)[on the transmission output shaft], except for a newer 22-tooth silver plastic gear, designed to handle the smaller od drive gear.

[Joe L.]

Re: Speedometer gear & odometer

David-----

As Michael and Wayne have mentioned, you've got the wrong gear installed for your application. Assuming that the transmission is original and has the original speedometer DRIVE gear installed, the 20 tooth blue gear is not only the wrong tooth count but it's from a series of DRIVEN gears that are not compatible with the installed DRIVE gear. Many "klutz" mechanics don't know that, though, and they believe that all speedometer DRIVEN gears are "interchangeable". The wear you found on your gear is consistent with incompatible gears.

The gear you require, as previously mentioned, is a 22 tooth GREEN gear of GM #3860345. This gear is still available from GM. Given your 9%, or so, inaccuracy, the 22 tooth count DRIVEN gear should get you as close as you're going to get to factory spec speedometer accuracy without going to an adapter. A custom calibrated adapter is usually the only way to obtain accuracy better than 3%, or so. This sort of thing is usually done only for police cars with "certified" speedometers.

[David K.]

Re: Speedometer gear & odometer

Thanks for all the advice. It is nice when there is a consensus. My 22 tooth green gear is now on order.