Same Muncie drive/driven speedo gears on '67s w/3.55 and 3.70

[Wayne M.]

Anyone notice (notice/complain) about this back when their cars were new ?

From Chevrolet Dealer Service Info Bulletin (blue), dated 23 March 1967:

With the 7.75 x 15 tires, a 3.70 ratio calls for a green plastic 22-tooth driven gear (3860345), coupled to an 8-tooth steel drive gear (3708145). This is the same as for the two previous model years which also used 7.75 rubber.

But the same combo drive/driven is quoted for the 3.55 final drive ratio. so one or the other will be off calibration by ~5% [or both, partially, to a sum of ~5%].

Prior to 1967, the 3.55 took the red plastic gear (21-tooth) with the other 8-tooth steel drive gear (#3708144).

I notice there were other changes in '67 (ie. the 4.11's in prior years took the yellow 24-tooth plastic gear; but in '67, they call out the 25-tooth orange). And there's more.

Was there something different in the '67 7.75 x 15 tire, compared to '65 and '66 ?


c

[Gene M.]

Re: Same Muncie drive/driven speedo gears on '67s w/3.55 and 3.70

Wayne,
That is interesting. It would seem the 3.70 gear will be reading faster than actual and the 3.55 will be reading slower. Also the 3.70 gear will be racking up miles faster than the 3.55 geared car. But if you face the 5% difference as a split, as in one faster and the other slower it is only 2 1/2% error. So 60 MPH true is 1 1/2 MPH off. The 3.70 reading high (61.5) and the 3.55 reading low (58.5). That is not much. One would have to do the math and account for tire deflection on road to verify tire rotations per mile to determine if there is an error and where.........

[Joe C.]

Re: Same Muncie drive/driven speedo gears on '67s w/3.55 and 3.70

Probably along the same lines as stuffing BON-AMI into the carburetors of 1955 shoeboxes as a cure for bad rings. Or sawdust for ring/pinion issues.

[Chris D.]

Re: Same Muncie drive/driven speedo gears on '67s w/3.55 and 3.70

From a few years back, looks like the 7.75 tires rolled a bit faster....https://www.forums.ncrs.org/archive/...p/t-90765.html

[Duke W.]

Re: Same Muncie drive/driven speedo gears on '67s w/3.55 and 3.70

When you have 20-something teeth you can only change the calibration in 4-5 percent increments, so it was a judgment call by the engineers.

The 6.70-15 tires up to '64 were speced at 760 revs/mile, and the 7.75-15 are 776, so that's about two percent right there, and revs/mile increase about two percent between new and wear to the tread bars.

The engineers probably decided it was better to have the speedo read a little high than a little low. There were no standards back then, but they were later promulgated by DOT, and IIRC there is now no allowance for low readings, but they can be a couple of percent high.

Modern electronic speedometers are typically dead-on with new tires of the correct size, but a little high side error is picked up as the tires wear.

Things were a lot simpler, but cruder back in the sixties.

Duke

[Gene M.]

Re: Same Muncie drive/driven speedo gears on '67s w/3.55 and 3.70

I prefer the car read slower than actual. It will rack up less miles on the odometer. Just be aware of any difference by compare cars speed by using the mile markers or a gps.

[Terry M.]

Re: Same Muncie drive/driven speedo gears on '67s w/3.55 and 3.70

When you have 20-something teeth you can only change the calibration in 4-5 percent increments, so it was a judgment call by the engineers.

The 6.70-15 tires up to '64 were speced at 760 revs/mile, and the 7.75-15 are 776, so that's about two percent right there, and revs/mile increase about two percent between new and wear to the tread bars.

The engineers probably decided it was better to have the speedo read a little high than a little low. There were no standards back then, but they were later promulgated by DOT, and IIRC there is now no allowance for low readings, but they can be a couple of percent high.

Modern electronic speedometers are typically dead-on with new tires of the correct size, but a little high side error is picked up as the tires wear.

Things were a lot simpler, but cruder back in the sixties.

Duke

[Duke W.]

Re: Same Muncie drive/driven speedo gears on '67s w/3.55 and 3.70

I recall reading somewhere that current DOT speedometer accuracy standard is +2/-0 percent from true speed.

I also recall that the GM standard for speedometer cable speed is 1000 RPM @ 60 MPH. A good speedometer shop can alter the magnetic flux in the speedo head to alter the gain error a small amount if a change in output gear tooth count is too much. I had that done to my Cosworth Vega, which has shorter than OE tires, so the speedo is now dead on at 70 MPH.

I had the same thing done to the tach - had the shop alter the flux to make it dead on at 7000 revs, but that means it now reads 200 low at idle, which was okay with me. I wanted best accuracy at the top end. Prior to the change it was dead on at idle but about 200-300 low at 7000. So all those years I through I was shifting at 7200, it was really barely 7000.

Duke

[John H.]

Re: Same Muncie drive/driven speedo gears on '67s w/3.55 and 3.70

On most modern cars, the speedo calibration is done automatically as part of the Roll-Test operation; the roll-test computer knows what tires the car has on it, the diameter of the rolls is a constant, and at some point during the roll-test sequence, the roll-test computer and the car's ECU exchange the necessary algorithm, and the speedo is dead-on accurate from then on, based on firmware. In the roll-test software (at least in Chrysler plants), that step is called "program speedo pinion".