Speedo Drive

[decoupe]

I drove into the city today - car runs like a champ 3000rpm = 70mph. Drive from place to place in town with no issues and the tunes blasting away. Leaving the city - heading out to the flats towards the mountains with someone in an X5 waving and pointing when I noticed the speedo reads 90mph at 3000rpm!! Uphill. There's a strong tail wind but nothing is going to give those numbers unless the tailwind is direct into the engine (as in turbo). And everyone else is moving at the same relative speed (well, actually slower).

So there's a problem here. The needle climbs smoothly and is steady at any reading but is consistently 15-20% high for the engine speed. No new noises. No pools of fluids on the ground. Shifts fine.

Possible culprits are the speedo drive, speedo cable or the speedometer. Speedo rebuilt in 2000 and only 37,000 mi since then. I think the cable either works properly or not at all if there is a problem. I'm guessing it's the speedo drive in the transmission. Can that cause problems?

This isn't a big issue since the speedometer is in miles and Canada is metric (kph) I tend to drive by the engine speed and gear but this will have to be dealt with at some point.

Any advice is welcomed.

Doug

 

[Bill Riblett]

The speedo drive is really just a couple of gears - not really anything that could cause your problem. I'd suspect the speedometer. They often weren't very accurate even when new, although not as bad as yours!

Have you changed the diff ratio, or changed to larger diameter (taller) tires?

 

[decoupe]

All of this happened today so nothing to do with the diff ratio or tires etc. Literally everything normal and then the speedo is out by 20%. Something "broke".

I'm almost hoping that it is the speedo as it would give me an excuse to convert to metric at the same time.

 

[Bill Riblett]

Pretty much has to be the speedo, none of the other parts are going to change like that.

 

[Honolulu]

I can't imagine it

How could it read smoothly and go 20% high? Either they work or they don't.

The "usual suspects" are North Hollywood Speedometer, and Palo Alto Speedometer, but these are in California. Don't know anyone in your neck of the woods who would do this work but surely the list will come up with suggestions. There are others that do not come to mind just now.

It might be as simple as a cleaning but really this is a puzzler. Are you confident/competent to remove the speedo and have a look? If not, let it be.

 

[decoupe]

North Hollywood did the rebuild last time - I'll give them a call after I put the car on the hoist and check the cable at the tranny. I'm good to remove the speedo but I'll leave it until the fall.

If the error is constant and increased another 10% I wouldn't have to convert mentally to kph. How's that for optimism.

Thanks all.

 

[Arde]

While I was having my speedo fixed I put a temporary one I got
on ebay that did go wildly up. It was correct at really low speeds (30 MPH)
and then it would shot up and even oscillate. I opened it and nothing
obvious was wrong, so it has to be in the gears.

My working speedo reads about 4mph high for most speeds. Not a percentage, an offset, which is good. One could just rotate the
mechanism with respect to the dial and take care of it...