67 3.70 rear (4 speed) - Speedometer (driven) gear information needed...

[Bill C.]

hey all ...

Quick question (i hope)..
my re-calibrated/restored speedo is reading 5 MPH slow to actual.

I am pretty sure I need a new driven gear and if I reduced the tooth count by 1 or 2 it should fix things up.
what i don't know is if that gear exists.
The drive gear is factory installed for the 3.70's.

Am I pretty much SOL or is there a gear available?

---- i really don't want to remove the dash to get the speedo out and send it back etc... -----

Thanks in advance - Bill

[Edward J.]

Re: 67 3.70 rear (4 speed) - Speedometer (driven) gear information needed...

Hi Bill, Long Island Corvette.s Web site shows all the gears and the number of teeth and what gear is correct for the 370 Ratio. if you subtract a tooth it should speed up. normal is 22 green for 370 gears, providing the drive gear is correct.

[Bill C.]

Re: 67 3.70 rear (4 speed) - Speedometer (driven) gear information needed...

thanks Ed..

greatly appreciate the info..

Bill

[Joe L.]

Re: 67 3.70 rear (4 speed) - Speedometer (driven) gear information needed...

Hi Bill, Long Island Corvette.s Web site shows all the gears and the number of teeth and what gear is correct for the 370 Ratio. if you subtract a tooth it should speed up. normal is 22 green for 370 gears, providing the drive gear is correct.

Edward and Bill------

If one "subtracts a tooth" from the 22 tooth green, one would obviously end up with a 21 tooth gear. The problem is that there is no 21 tooth gear available which would be compatible with the DRIVE gear used with the 22 tooth GREEN driven gear. So, if you want to install a 21 tooth DRIVEN gear, you will also have to change the DRIVE gear (which, of course, is not easy to do).

By the way, if your car has a 3.70:1 rear gear ratio and has tires of the original OD (or, revs per mile), then using the 22 tooth GREEN driven gear should give you a speedometer reading as accurate as when the car was new. Of course, that's not necessarily mean 100% accurate.

[Wayne M.]

Re: 67 3.70 rear (4 speed) - Speedometer (driven) gear information needed...

Here's a chart from a 1967 Tech Service Bulletin that I find useful. You'll notice that for '67 Corvettes with 7.75 tires, both 3.55 and 3.70 diffs used the same green 22 tooth driven gear. P&A 30 parts books confirm. Code 5C, part # 3860345.

I'd be curious to know if actual production followed this.

[Bill C.]

Re: 67 3.70 rear (4 speed) - Speedometer (driven) gear information needed...

Wayne ...

Is there any chance I can use a 21 tooth gear with the 3.70-4.56 drive gear?
if they mixed the 22 tooth gear with two different drive gears, maybe I have a chance.

Bill

[Joe L.]

Re: 67 3.70 rear (4 speed) - Speedometer (driven) gear information needed...

Wayne ...

Is there any chance I can use a 21 tooth gear with the 3.70-4.56 drive gear?
if they mixed the 22 tooth gear with two different drive gears, maybe I have a chance.

Bill

Bill------

Absolutely no chance, at all. At least, not if you want the driven gear to last more than a few days.

By the way, there are two different 22 tooth driven gears-------the green 22 tooth and the silver 22 tooth gear. The silver gear did not even exist in 1967; it came along about 1971 (It can be used in a 1967, though, as long as the transmission is also equipped with the 1.85" OD drive gear) In any event, since you have the green 22 tooth gear currently installed, that means you also have the 1.77" OD drive gear installed. That drive gear is only compatible with the 22 tooth green through 25 tooth driven gears.

[Bill C.]

Re: 67 3.70 rear (4 speed) - Speedometer (driven) gear information needed...

well ....
as much as I hate having to pull the dash cluster again....
seems only real choice here.
fortunately - since I have done it very recently, not going to be a horrible thing...

I'm going to get a months worth of driving first, the sideline the car when the weather get cooler in DEC.

Thanks for all the great feedback .

Bill