Speedometer problems
 

When we first got the GTO back together, we found that the speedometer read slow. Over the years the car had various sets of gears in the rear end so this wasn't a huge surprise. The car originally had 3.23 gears but we ended up with 3.55s. Anyway, based on which driven gear we had I did the math and determined which driven gear we needed, I changed the driven gear and everything was great. The speedometer was right on based on timing between mile markers and speed read through a GPS app on my phone.

However, when I got the GTO out for the first time this past weekend, the speedometer barely read at all at first, was really slow to react to changes in speed up or down and basically acted poorly. As the drive went on and things all got properly warmed up it got better, but still read about 5-10% slow. When speeding up or slowing down it occasionally appears to temporarily stick at 40 mph or so so something isn't right. No noise from the cable or anything and it has been properly greased. Would this be an issue of the speedometer itself and is there anything I can do myself to address the issue? It was clean when it went back in and I don't suppose you are supposed to lubricate anything in the speedometer but then I've never had much trouble with a speedometer. I could try cleaning and regreasing the cable, but I don't see how that would be the source of the problem.

Does the odometer measure miles approximately correctly? If so then it has to be an issue in the speedometer.

All I can confirm is that the odometer is moving, I haven't checked it for accuracy against mile markers.

Sounds like you need your speedo head rebuilt. Pete Serio Is your man to fix it.

Sometimes little critters make their home in there.

George

I'll be giving it the once over myself first, but if that doesn't do the trick, how do I contact Pete?

Pretty sure this is his contact: PrecisionPontiac.net. He does excellent work. If you have questions call him. Very friendly guy.

Or you can send him a PM , he's on the forum here. He did mine in my car, excellent work and extremely knowledgeable.

Okay, I got the speedometer out of the car. What fun! Nothing got broken or scratched at least. But it will be a pain to put it all back together.

Anyway, with the speedometer in hand, nothing looks amiss. The needle bounces smoothly and with a drill on the input it moves the needle just fine though given how fast it moved the odometer, about 3 tenths in just a few seconds, I'd say it is reading very slow as it never got above 50 mph. I also plan to send a PM off to Pete as mentioned above.

As far as calibration goes. There is a mathematical formula that all 1950s, 1960s, 1970s and 1980s GM car speedometers used. If it was a front engine; rear wheel axle drive. It goes by the RPM of the input cable. That is: 500 RPM on the input cable = 30 MPH. 1,000 RPM on the input cable = 60 MPH and 1,500 RPM on the input cable = 90 MPH. The speedometer is driven by Cable input into a a grey iron magnet (thru an air-gap) to move the MPH indicator. The odometer (total miles) counter is gear driven by the speedometer cable input.

Anything over 100,000 miles or 25 to 30 years and the speedometer is at the end of is designed in lifespan & should be rebuilt.

They were never made to be field serviced and they were intended by GM to last at least as long as the car would run from the time it was a brand new car up until 100,000 miles. This assumes regular use, with the car driven at least 3 or 4 times per week. That, or as a daily driver.

I often see older GM car speedometers that came a car that has less than 100,000 miles on it. Except that it was from a car that was driven and then parked. Often for many years never started nor driven. Letting a car sit in storage the factory oil will deteriorate. In those cases lack of use causes the oil inside to congeal and then when you take the car out again the pointer, binds, sticks and then it jumps. It will make a chattering noise when the oil inside has turned into sludge.

That is a sure sign the speedometer needs completely torn down, all parts cleaned & inspected underneath a magnifying glass and anything worn out or dried replaced. At the very minimum a new bushing should be installed and then the iron magnet re-magnetized and calibrated.

I use a 24 volt DC powered set-up to re-magnetize the iron bar. After that I use a tester made by Stewart Warner especially for putting known RPM on my cable. You could also use a drill assuming that you have a way to know the RPM. I have several short pieces of cable here with the .104" square drive on one end that I use as a tool when I test-run one using a 1/4" drive Makita drill.

Peter, thank you for the great information. Very cool Stewart Warner tester. I know that nobody would notice, but just to be mathematically exact, isn't the number 1001 revolutions per mile or 1001RPM = 60MPH?

1001 RPM at 60 MPH yes for the odometer to count over 1 mile. We are not dealing with something calibrated to land on Mars.

None of the passenger car speedometers were ever that close to start with. In fact if you look into any of those old Road & track or Autoweekly magazine road tests you will see that GM delivered all new cars with the speedometer's reading greater than actual. Over the life of the car the magnetic strength looses energy so each year the speedometer would read a little less MPH. Often they were magnetized up to strength to read 10 or 11 MPH greater than actual. During the "life" of the car each year the magnet (actually a piece of grey iron) would loose a tiny bit of it's strength so you can in all of those old road tests where they put a Weston Electric speedometer in the car ( a 10 speed bicycle wheel & tire) clamped to the rear bumper. All GM cars & trucks, their speedometers were calibrated to read higher than actual in the beginning and then over the normal life of the car they settle down bit by bit.

Back then it was expected the car would run for about 10 or 12 years ( driven every day) and in that amount of time the car would accrue at least 100,000 and perhaps more miles.

And look at the (lack) of precision for these GM speedo adjustments: change the trans driven gear by one tooth. The range of most gear teeth choices is around 20 or so, so you are making approx 5% changes when you correct there. No way the math of 1000/1001 comes into play. Tire brand height variations can push you quite a few % one way or the other too.

So if your dash is anywhere _near_ your GPS, celebrate!

I just swapped from 3.55 open to 3.23 posi, did the math and formula said swap existing 22 with a 20 tooth gear. But since my speedo was already reading a bit high with my tires (Coker redline G70’s) I put in a 21 tooth and now it is within approx 1 mph at cruising speeds.

Victory!


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Speedometer will be on it's way to Peter very soon.