Usually wrong speedometer was right for a few minutes

[DarkFlounder]

Usually, the speedo on my 72 K2500 is off by a significant amount (showing 35 when I'm going 65). I always figured this was due to the larger tires (35s) and a possible rear axle gear ratio change (PO swapped a Dana70 in place of the Dana60).

However, driving home yesterday, the speedo suddenly jumped and started showing almost the correct speed (showing 61 when I'm doing 65). It maintained this until I slowed for the offramp, and suddenly jumped back down to its usual incorrectness.

Could my speedo actually just need a good cleaning and lubing?

[KRamsey07]

when they are off its not linear through the whole speed range . What i mean is at 60 it might be off to say 55 or less but it wouldnt be going 5 at 10 10 at 15 and so forth . As to what your problem is im not an internet diagnostic mechanic i am sure lots other will chime in . I hate to throw parts at something to fix it and id rather see it before i make that call. Hope this helps and i hope someone leads you to the right issue !!!

kenny

[EARNHARDT#3]

Try lubing the cable first.

[swamp rat]

Humm, the speedo cable would jump around rather fast if it needed lube, as in every revolution so it would only vary a few rps per revolution and not be a steady reading at a different speed.

The only thing i could think of would be the drive mechanism is geared wrong but that wouldn't allow it to suddenly speed up.

Are the ends of the cable still squaired and not rounded?

[pritch]

Was it really cold outside? Mine is usually pretty close to correct, but when it's real cold it'll say I'm doing a hundred when I'm just going 30 or 40.

[68gmsee]

If after checking the drive and driven gears inside the tranny they look okay and not stripped then another possibility is the speedometer mechanism in the dash panel. It's basically a rotating magnet that creates "eddy currents" and the stronger the current the more the needle moves. Something could be loose or defective in there.

[68 TT]

Originally Posted by EARNHARDT#3

Try lubing the cable first.

I second that. I have fixed several misbehaving speedometers over the years with just a good cleaning and grease job on the cable. Soak the sheath and cable in mineral spirits for a few hours then blow it dry to remove any debris or crud. After that use white lithium grease on the cable then put it back into the sheath.

You can also spray the speedo mechanism with electronics cleaner to remove any crud that may have accumulated on its moving parts.