Odometer error

[Theodore K.]

My 1957 Corvette has had a long standing odometer (25yr) error of about 20%. Don't know why. I recently changed the rear end from 3:08 to 3:36 and changed the speedo drivengear from 18 to 20 Tooth. The speedo always has been indicating about 5MPH faster than actual. With the read end change that increased to a 10 mph error. The change of driven gear was to compensate for that error.The Speedometer is now right on at 55 thru 70 MPH. Recently I checked the odometer against the highway measured mile and found it indicated 1.2 miles traveled for only 1 mile actual.

I am wondering if anyone has encountered this same problem and what can be done to fix it if anything.

comments are welcome?
Ted von Kampen

[Jack H.]

Re: Odometer error

Both the speedo and odometer are driven from the same input shaft. The difference is the odometer simply counts upward while the speedo drives a pointer needle that 'stands' at various points depending on the specific velocity of the vehicle.

In order to make that (speedometer pointer needle) happen, there's an air gap between the rotating input shaft and the pointer needle drive with the two magnetically coupled to each other and an appropriate degree of mechanical damping on the pointer needle to prevent needle 'bounce'.

Speedometers are calibrated by adjusting the specific distance of the air gap AND the field strength of the coupling magnet. Those speedo shops that were certified by Delco had the same final calibration/test equipment that was used by the factory. We've got such a shop here in Denver.

They'll put the speedo on the money via air gap and magentic field strength adjusment. Then, your reporting errors will all relate to your selection of driven gear, wheel and tire size...

[Ken A.]

Re: Odometer error

Jack,
There is no "air gap" adjustment. All adjustments are done via the hair spring location and a magnetizer/demagnatizer. If the odometer is original, it is most likely worn out or damaged by "adjustment" from a previous seller(s).
Ken

[Richard G.]

Re: Odometer error

FYIO
If I remember correctly speedometers in general are designed and calibrated to read 60 MPH at 1000 RPM at the cable.
Rick

[Joe R.]

Re: Odometer error

The best advise would be to have your speedometer checked by a specialist in Speedometer Repair. They should do it for free and give you advise based on what they find.

You should always start your troubleshooting with known calibrated instruments and parts so you will have a reference.

Note: 60MPH is 2000RPM on a 1 to 1 ratio calibration machine.

JR

[Joe R.]

Re: Odometer error

Sure it isn't 1,001 RPM Joe? I found this chart all over the net. Doesn't make it correct, but I've always thought 1 mile = 1,000 revolutions of the cable. Not 2,000 So 60MPH would be 1,000 RPM, if I'm correct. Are early Corvettes different?

Dave,

Just quoting what's on the dial of my calibration machine. Yours might be different but I get by using the most accurate machine ever made. A model 890 F. W. Stewart.
JR

PS. We're talking about the cable speed at the back of the speedometer. Not the RPM of the motor, driveshaft or drive gears. The cable speed would be the speed of the driven gear.