dsafian

Hi,

I have a '63 327/340 w/4spd that I just picked up. At low speeds 35mph the speedo is accurate, but the faster I go the more the variance.
At 50 - 60 it's +10 mph, and when I'm driving 70, it reads close to 90mph or +20 mph. I'm running 205 / 75R15 tires.

What's the best method for correcting - I'd rather not put more miles on the clock than necessary!

Thanks.

DansYellow66

That's a pretty sizeable speedo error but I was guess that the speedometer gear just needs to be changed. First thing to do is climb under the car, take the speedometer cable loose at the transmission, remove the retainer bolt and remove the speedometer gear housing and plastic gear. They just pull out once the bolt and retainer are removed. But the o-ring will provide a little resistance so you will have to work it out a little. Count the teeth on the gear and report back with that information and the color of the plastic gear. Also, do you know for sure what your rear gearing is? If not you should try to determine that too. You will get some help then with selecting a new gear.

There is a possibility the speedometer head up in the instrument panel is out of calibration - but try the easy stuff first with the speedometer gear.

kellsdad

If the speedo gear is wrong (possibly due to tire size or rear gear change), then the error will be proportional at all speeds. For example, if it is 5 MPH fast at true 30 MPH, then it will be 10 MPH fast at true 60 MPH. If the error is not proportional to speed, then I would suspect either a problem with the speedometer head or possibly some slippage somewhere in the gear/cable. ..... I drove my car for years with a known error of 10%. For example, I knew that when the speedometer showed 60 MPH I was really doing 66.

dsafian

DansYellow66 - thanks! I figured this was the place to start, but wanted to get a little feedback. I don't know the ratio, as the car was restored 20 years ago, and it seems to have other minor modifications - Cam, etc.. Will go through the paperwork to see if I can shed more light on that. Will pull the gear and report back. Cheers!

dsafian

kellsdad - thank you - I have to look closely at the margin of error at all speeds to verify. I think it's consistent, but way more that 10%.
I would probably leave it, but don't want to rack up the added miles.

61corv

MOXIE62

The original plastic gear is brown and has 18 teeth. If you want the speedometer to go slower you need the next size which is black and has 23 teeth.

claf

It's unlikely the gear is the wrong one if the speedometer is accurate at one particular speed. I have the same problem with my '62. Waaaay off at low speeds but the faster I go the closer it gets to correct. My problem is some kind of imbalance inside the speedmeter due to some soldering inside the speedometer from a way back when "bubba" repair.

Do not worry about the miles (if in fact you have the correct gear) as there is no direct relationship between the odometer and the speedometer. Odometer is gear driven, speedometer is magnetic driven.

SWCDuke

There are two kinds of speedometer error - offset error and gain error.

Gain error means that the percent error is constant at all speeds. Offset error means the speedometer is off by constant MPH at all speeds.

The limited data you've provided indicates you could have a combination of both offset and gain error, but more data is needed.

Take a set of readings in 5 or 10 MPH increments from 15 MPH to your highest freeway cruising speed and compare them to either a portable GPS in your Corvette or the readings of a modern car pacing your Corvette. Post the data.

Gain error can be caused by incorrect speedo gearing for the installed axle or tires with significantly different revs/mile than OE, but your installed 205/75R15 tires are close enough to OE tires revs/mile that they will not introduce significant error. Gain error of more than 3-4 percent can usually be corrected by installing a different speedo driven gear, which will change the reading up or down by about five percent.

Offset error can only be corrected by removing the speedo needle and reclocking it on the pin.

In addition to the speedo/GPS data you should read the axle code that is visible between the strut rod bracket and spring, and you may have to do some cleaning to see it. Also, count the number of driveshaft revolutions for one axle revolution to verify the installed ring and pinion ratio. It may be different from the code that shows the original axle ratio.

There's no way anyone can diagnose and recommend a solution to your problem until you provide a complete set of data as discussed above.

Duke