speedometer function - Accuracy - The 1947 - Present Chevrolet & GMC Truck Message Board Network

RyansToy · 12-09-2015, 04:18 PM

So I took my 68 out for a longer trip before I put it away for the winter. My buddy followed me to the storage spot (~250 Miles).

I have never checked the accuracy of the speedometer with GPS or a friends vehicle till now.

Odd thing was that at 50 MPH the speed appeared accurate,
But at 60 or 70 MPH the speedometer is ~30% low.

How does the speedometer work? does the mechanical cable turn a friction mechanism to show the speed? or does it turn a small generator to move an electrical gauge?

I would have thought the speedometer would be either correct, or wrong, but not both....

Wrenchbender Ret · 12-09-2015, 05:16 PM

The cable turns a small magnetic drum that that pulls the needle armature against spring pressure. It does seem strange that it is not even like that. First check the odometer against the mile markers on the freeway. If that is close, your problem is in the speed indicator.

MagmaJct · 12-09-2015, 05:16 PM

If it's off as a percentage of actual speed, then it'll be less apparent at lower speed, and more apparent as higher speeds.

Regards,
Mike

kehstr · 12-09-2015, 06:08 PM

If the needle is still good on your speedo, i will trade you for a working one i have, that i'm going an electronic speedo conversion, and just need the needle to complete the conversion. Pm me if you are interested

12-09-2015, 04:18 PM	#1
RyansToy Registered User Join Date: Aug 2015 Location: Nova Scotia Canada Posts: 145	speedometer function - Accuracy So I took my 68 out for a longer trip before I put it away for the winter. My buddy followed me to the storage spot (~250 Miles). I have never checked the accuracy of the speedometer with GPS or a friends vehicle till now. Odd thing was that at 50 MPH the speed appeared accurate, But at 60 or 70 MPH the speedometer is ~30% low. How does the speedometer work? does the mechanical cable turn a friction mechanism to show the speed? or does it turn a small generator to move an electrical gauge? I would have thought the speedometer would be either correct, or wrong, but not both.... __________________ 1968 GMC truck 910 Trim (Canadian) 127" WB 3 on the Three Trans (Saginaw SM330) 250 ci L6 Engine HEI ignition (early 80s L6)

12-09-2015, 05:16 PM	#2
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,204	Re: speedometer function - Accuracy The cable turns a small magnetic drum that that pulls the needle armature against spring pressure. It does seem strange that it is not even like that. First check the odometer against the mile markers on the freeway. If that is close, your problem is in the speed indicator.

12-09-2015, 05:16 PM	#3
MagmaJct Registered User Join Date: May 2008 Location: Mesa, AZ Posts: 281	Re: speedometer function - Accuracy If it's off as a percentage of actual speed, then it'll be less apparent at lower speed, and more apparent as higher speeds. Regards, Mike __________________ I own 3 classic vehicles. 1971 LWB C10 Deluxe. 1974 VW Super Beetle. 1971 VW Fastback. The C10 and the Super Beetle run, the Fastback is just not there yet! Please visit my Super Beetle and Fastback profile on TheSamba.

12-09-2015, 06:08 PM	#4
kehstr Registered User Join Date: Mar 2015 Location: Utah Posts: 750	Re: speedometer function - Accuracy If the needle is still good on your speedo, i will trade you for a working one i have, that i'm going an electronic speedo conversion, and just need the needle to complete the conversion. Pm me if you are interested



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