Speedometer woes

I have a 1990 Buick Skylark with 95K miles on it and the speedometer does not work. The tripometer and odometer seem to work fine, but the speedometer is NEVER anywhere near right. Any suggestions on what might be wrong with it? How expensive it will be to fix (we’re DIY’ers) and how much of a pain it will be would also be helpful information.

Also, if you have any clue, what is the proper tire size for this car? The sticker on the door has faded away and the owner’s manual simply says “Check the sticker on the door.” They’re 13" and half the tires are one size and the other half are another.

I think the tires are supposed to be 14" a on the Skylark.

Sorry I can’t fix your speedo tho.

Curious tho, how do you know it’s not right? Have you tested it against a radar gun or tracked another vehicle at a pre-arranged speed?

That model could have had either 185/80R/13 or 185/75/R/14 tires

Different size tires can throw the speedometer off in their own right plus there may be potential handling issues as well.

First you need to go to your local Buick dealer and find out exactly what size your rig takes and get the same diameter tires all the way around.

Then I would hit the interstate and pull a measured mile to see if the odometer is indeed accurate. THEN, I would find a trooper with a speed gun and have them clock you at a pre-determined speed to see just how far off the needle is. If the odometer is accurate and the needle is still off with the correct tires, then the speedometer head is bad and needs replacement. If both the odometer and the needle are off, then I would lean towards a possible speedometer cable issue as well. Chuck

Yes, you may be able to fix it yourself. You have to remove the instrument cluster and take out the speedometer.

The instrument is quite simple. Most problems are due to one or more worn gears. Determine the size and number of teeth on the gear(s) that need to be replaced. There are internet companies (use Google) that can replace any gear.

You may even get lucky (as I did when fixing my Mercedes speedo) and find that one gear is simply not turning the shaft on which it rests. A drop of Krazy Glue took care of that.

I know the speedometer’s off because it’s GONE off the dash before I’ve gotten to 50 or 60 MPH. It goes past 85. And I can gauge my speed in town pretty well, because I’ve traversed the roads of where I’m from my whole life, and I know I’m not doing 50… I’m doing 30. It’s not off in a wrong-size-of-tire kind of way though. My sister-in-law’s Volvo has the wrong size tires on it, and it’s off by 3 MPH, we’ve radar-gunned it.

Have you replaced the transmission recently? We had a similar problem with an '88 Buick when we replaced the tranny. This car uses a Hall-effect sensor to determine the speed. The sensor is located on the passenger side of the transmission right near the inboard CV joint. The rotor is under the cover. I’ll bet the rotor has more ‘fingers’ on it than the original transmission, and the speedometer is mis-calibrated because of it.

If you can get the original transmission, just swap out the rotor. Otherwise, a trip to the junkyard may be in order. Or to a transmission shop. The cover only has 2 or 3 bolts on it, and is really easy to get to, right on top.

I shall check and see if that’s the case. My father gave me the car, after buying it from someone for my sister. She says the speedo hasn’t worked the entire time, and I know my dad didn’t do any tranny work on it. We’ll see.