Mechanical speedo says I'm going faster than I actually am

A short while ago I'd have this problem where the speedo would fly up way above the speed I was obviously travelling for a short while and then go back to normal. On the way home yesterday, it was consistently telling me that I was going about 20-30% faster than I actually was.

Could a worn speedo gear cause this? The C52 gearbox has a known design flaw where the plastic gear on the diff that drives the speedo gear is made of a softer (or roughly the same durometer) plastic than the speedo gear itself. I replaced the speedo gear maybe 6-7 years ago, but when it failed that time the speedo started going wobbly and eventually just stopped reading any speed at all.

So speedo gear problem or instrument cluster problem? The gear on the diff that drives the speedo gear is replaceable, if I can find a replacement. If I have to open that box up I think it's time for a rebuild though.

A cable drive speedo spins a doodad which uses magnetism or magic to make the needle go up. If the bushing is dry in the head, it transfers the magic directly to the needle, making it go too high. Back in the olden days, there were magic shops in most cities that could clean, lube and calibrate speedometers. Perhaps there is a wizard remaining in your part of the world.

If you look at it yourself, use a very small amount of lube, and I don't know what sort.

The only thing a speedo drive gear can do is fail, which makes the magic stop.

Ah that's probably it. I was going to replace the instrument cluster anyways, might just have to do it earlier than planned and get the replacement serviced before it goes in.

Mine did the opposite. At low speed it was close enough but 60 mph was really more like 70. The grease for the needle movement had dried into snot and dragged the needle enough for me to have a chat with officer friendly. A thorough cleaning and relubing of the speedo and a mostly successful court date ensued....

Cable drive speedo's, are there any left, often let out a squeal and jumpy needle when in need of lubrication. Once uppn a time, greasing the cable was routine maintenance.

iceracer wrote: Cable drive speedo's, are there any left, often let out a squeal and jumpy needle when in need of lubrication. Once uppn a time, greasing the cable was routine maintenance.

i've had many speedometers ( i refuse to use the abbreviated form of the word in this context..) that bounced around silently because the cable needed some lube. it's usually easy to do- pull the speedometer out, unhook the cable, and pull the inner cable out of the outer sheathe.. then put some speedometer lube in the cable and slide the inner back in, spinning it by hand as you push it in.. put it back together and go for a drive and the needle should be nice and steady.

My Jetta gives me erroneous reading (Cop paperwork to verify) due to oversized summer wheels-tires. I use the GPS for speed control, or just let my wife punch me in the thigh.

Dan

I would like to think powdered graphite would work well in this situation.

Appleseed wrote: I would like to think powdered graphite would work well in this situation.

it does.. and that's what the speedometer lube is made from..