63-67 Speedometer-Odometer

[Gerard F.]

Was the original gearing for the 63-67 Speedometer Odometer 1000 revs per mile, or was it some other factor?

Jerry Fuccillo
#42179

[John H.]

Re: 63-67 Speedometer-Odometer

Jerry -

The SAE standard for mechanical speedometer calibration is 1000 input revolutions per mile for the odometer (fixed mechanical relationship within the instrument from the input through the gear train to the odometer wheels), and 1000 input rpm = 60 mph indicated for the magnetically-driven cup that carries the needle. Accuracy of the latter is affected directly by tire size and axle ratio, and is compensated for by the ratio of the DRIVE gear on the output shaft vs. the DRIVEN gear on the end of the speedo cable to get as close as possible to 1000 cable rpm at an actual 60 mph.

[Gerard F.]

But, was that the 63-67 standard for

for Corvette speedometers? Honest, John I'm not trying to be a wise guy. I'm just trying to understand how someone (as in a recent post) reported that after changing the speedometer head and odometer gear, he had a speed reading of 20% low, and a odometer reading of 20% high. Shouldn't they both be on the same side, if it was only the cable speed involved. By increasing the cable speed via the driven and drive gears at the transmission to match the speed reading, wouldn't we be exaggerating the odometer reading?

Maybe the report was wrong, or I misread it, but I think this is worth some further research. Could the magnets against the cup be that far off (20%)in calibration.

Best Wishes,

Jerry Fuccillo
#42179

[Jon #40768]

Re: Elk Grove judging

Hi Jerry. I judged the interior of the 66 next to you last weekend. We talked about both of us having our cars for over 25 years. I have a question or two for you regarding the weekends judging. If you want you can respond here or off the chat line by e-mail.

Jon
lpeintl@comcast.net

[Michael W.]

I believe there's two separate issues-

An odometer reading is not subject to 'calibration' in the same sense as a speedometer. An odometer drive is a pure gear reduction 'machine' where X rotations of the transmission output shaft results in Y rotations of the odometer wheels. The appropriate gear reduction ratio (X/Y) for a given car is function of the differential reduction ratio and effective OD of the rear tires while sitting on the ground at standard inflation. If the correct ratio is chosen, (by varying the drive cable ratio at the transmission tail shaft), the output reading of the odometer is analogous to the distance the car has travelled, as indicated in miles in this instance.

If the indication is incorrect, and assuming that the odometer was repaired with the correct reduction gears, then the 20% error can only be induced by the use of the wrong reduction ratio in the transmission tail shaft and/or the wrong diameter rear tires.

All this to say that if the poster below were to correct his 20% odometer error, it would mathematically increase his speedometer error factor to 40%.

Can a speedometer be 40% off? Yes.

In my mind the fellow below has at least two separate problems.

[Gerard F.]

I agree

Mike,

I agree that there are two problems here, but I think the speedometer-odometer issue should be addressed first. The percent error in the speed reading vs the odometer reading should be the same, not opposite. With opposite readings, there is something wrong between the speedometer head and the odometer gear reduction.

If both the speedometer and the odometer are in synch with the same percentage error, then it would be time adjust the cable speed for the rear end and tire size by changing the driven and drive gears at the transmission.

Also, I'm not so sure that all older speedometers-odometers met the SAE standard of 1000 rpm @ 60 mph, or 1000 revs per mile. I'd like to do some further research on this, as a different standard between the head and the odometer gear reduction could account for Glenn's problem.

And then again, maybe his speedometer needle is 40% off.

Jerry Fuccillo
#42179