Speedometer issues

I did a bunch of work on my '65 Skylark this winter, including pulling out the speedometer in an attempt to lube it (it was clicking). It has always read fairly accurately. When I started driving it this spring, the speedometer was slow all the time, regardless of speed. I figured I screwed something up and sent the speedometer out to get rebuilt. It came back with no click, but it's still 10 MPH slow. I didn't mess with anything else with the speedometer, but I checked the cable, wiped it off, and relubed it after it came back.

I did get a new set of tires recently, of the same size, but I'm wondering if they could be that much larger to cause a difference. I would imagine, however, that it would be a certain percentage off at all times rather than roughly 10 MPH off at any speed. Either way, the odometer is accurate (as measured by mile markers on the freeway).

Any ideas? I did email my speedo guy (who is from a very long standing business), so I'm waiting to hear from him, but I have a hard time believing that he wouldn't check the calibration before sending it out. What else might it be?

Does anyone know where to get speedo driven gears for an ST300? Thanks, if nothing else, for letting me spitball here...this is a weird one. Sometimes, it's best not to poke the bear, that's for sure. I should have lived with a little clicking!

Check the odometer accuracy FIRST. If the "miles" are off a particular amount, then the "speed" should be similar percentage "off".

I suspect the OEM tire size for your vehicle would have been 7.75x14? Later to be F78x14 and P205/75R-14 in P-Metric sizing.

In the world of speedometers, there is a special relationship between the driven gear and the drive gear in the transmission tail shaft housing. This is what gets the "cable rpm" right. The cable then indexes with the back of the speedometer head, in the rearward section of the "speed cup". Inside the speed cup is a bar magnet, which can have its strength varied with a "zapper gun" at the speedometer shop to get the speed calibration the best it can be for the speeds they check it at.

Cable runs the back of the speed cup. As the magnet turns, it tries to turn the front side of the other speed cup, the one which has the spindle that the speedometer needle slides onto, a slip fit. As the back speed cup turns, it tries to turn the front speed cup, which is working against a small clock spring to make the speedometer needle return to "0" when the vehicle stops. Stronger magnetism makes the front speed cup better follow the rotation of the rear speed cup. Similarly, less-strong magnetism does just the opposite.

The KEY is getting the "miles" correct first, as that's a cable-driven function. You can do a somewhat accurate check with the mile markers on the Interstates, for a 10 mile stretch at a constant speed. With that done, then drive a constant 60mph over the same route, timing the time between each mile marker. 60 seconds = 1 mile @ 60mph . . . the famous "Mile a Minute" speed. For about 3 seconds either way from 60mph, 57 seconds would be 63mph, for example, as 62 seconds would be 58mph, or thereabouts.

OR . . . you could check distance and speed with a GPS device.

In order to verify your speedometer gears in the transmission, you'll need to find a GM/Buick Parts Manual which covers your model year. The speedometer drive and driven gears should be in a chart, or a lengthy listing with tire sizes and rear axle ratios. There will be a "tooth count" for each gear and GM part numbers.

After reassembling the speedometer head, the speedometer needle would be carefully reinstalled on the spindle of the front speed cup, placing the needle on a calibration mark on the speedometer faceplate. Once that basic position is done, then he'll spin it up with the speedometer speed check device. There should be three speeds that are checked. If some adjustments are needed, the "zapper gun" is used to "cut and try" vary the magnetism in the bar magnet in the rear speed cup. Once the "best fit" calibration is done, within the specs in the speedometer manual, it's done. The gear which runs the odometer is "fixed" and not serviceable, but the speedometer drive/driven gears in the transmission are.

You might be able to find sections of a Buick parts manual in the "tech" section of the www.wildaboutcarsonline.com website. The existing speedometer gears will have a specific color and number of teeth. The tooth number should be cast on a smooth section of the gear, too.

Checking the odometer first will tell you if the gears in the transmission (for the speedometer) are correct for what you've got. When they are correct for the tire size and rear axle ratio, then the speed portion should fall into place.

Please advise of your progress.

NTX5467

That's the confusing part...I checked the odometer against mile markers on the freeway last night, and it's basically accurate. One mile equaled one mile (very closely anyway). Usually I can work through problems like this, but I'm stumped!

What are your times for 5 miles at indicated 60mph? For the same 5 miles, what does the odometer indicate? It takes more than 1 mile to do the odometer check, from my experience. What does the odometer indicate for 10 miles @60mph? ONLY by going the longer distances and maintaining a steady 60mph can you really get the correct percentage of the error. If you need to drive faster than 60 to keep out of the way of oncoming traffic, that's fine, just that it you have different mile/seconds numbers than 60 seconds/mile @ 60mph.

One of my older cars has a speedometer that's dead-on for the odometer, yet reads 10% faster speeds than is actually being driven. I leave it be due to the difficulty of getting the speedometer head out of the instrument panel.

NTX5467

Well, I did the odometer test by driving 10 miles on the freeway today, and the odometer is actually .4 miles FAST over 10 miles, yet the speedometer is approximately 10 mph slow at all times. I think I'm going to have a heck of a time figuring this one out!

The magnetism of the magnet in the rear speed cup (of the speedometer head) is weaker than it should be. It just needs to be "zapped" with the "gun" at the speedometer shop (where ever that might be). It's a "cut and try" method . . . one zap might make it faster, another might make it slower, so you just have to get the "best fit". Of course . . . it could just be that the needle needs to be repositioned (gently twisted on the front speed cup's spindle, where it sticks through the speedometer face plate) if the speed is consistently slow across the normal speed range (no need to "zap" for this readjustment). ONLY thing is that you have to be VERY gentle as the spindle for the needle is (I believe) made of brass and is of VERY small diameter--proceed at your own risk.

From your numbers, the odometer is reading 4% fast (100 miles traveled indicates 104 miles). That same error would result in an indicated 62mph really being 60mph.

Seems like there is a speedometer cable vendor in MI, who has a very good reputation for correct speedometer cables for older GM cars? Perhaps he might know who might help you with the speedometer calibration?

Thanks for the update,

NTX5467

Just to update...I pulled the speedometer out, and I don't know how I didn't notice this, but the needle needed to be repositioned to zero MPH. It must have gotten tweaked in shipping or something. I have yet to reinstall it, but that would have to help...it was resting all the way over to the left on the speedometer face itself, so I had to move it at least a half inch. I just grabbed the spindle with a pair of needlenose pliers and moved the needle as gently as I could until it was positioned correctly. It seems to move and return normally, so I guess I didn't bend anything! My goal is to get it back into the car and try it again tomorrow. Thanks for awakening me to that idea, NTX...

Thanks for that update! Please keep us posted on your progress.

NTX5467