Swapped speedometer head, now different indicated speed

[Fbks_Cruiser]

I’ve been dealing with a chirping noise from the speedometer. I replaced the speedo cable when I got the truck years ago.. The noise just showed up intermittently last summer. I pulled the inner cable, cleaned and lubed it but still chirped on occasion. The last month or two it got more frequent and loud.

I also needed to replace the instrument printed circuit so I figured now is the time to try to fix the noisy speedometer since I was going to have to pull the cluster. I have a instrument cluster I pulled from a 1969 K20. So, I pulled the speedometer from it, rolled the odometer to match the mileage on mine, painted the needle and put it in my cluster. New printed circuit and bezel. Looks great! I also again pulled the inner cable, lubed it and verified no sharp bends, kinks or pinch points.

Took it out for a test drive today everything seemed to work great, nice and quiet with a steady needle. But, my ear and butt told me I was going faster than the speedometer said. I went back home and grabbed the GPS. The original speedometer read about 10% too fast. (60 mph indicated was more like 54-55 mph on the GPS). The new one reads about 15-20% too slow! ( 50 mph indicated was about 58 on the GPS). This difference agreed with what the “this is your speed” radar sign on the roadside said.

Is there really that much difference between factory speedometer accuracy?

BTW I’m running the NP205 transfer case and 235/85R 16 tires.

(The angle of the photo makes it look like the hash marks don’t line up with the numbers, but they do line up.)

[HO455]

Originally Posted by Fbks_Cruiser

Is there really that much difference between factory speedometer accuracy?

After 50 years. Yes.

Our Speedometers work by magnetism and over time and use the magnets can lose their magical powers and then the needle no long points correctly.
If he doesn't chime in P.M. TBONE1964 for the fix.

[Steeveedee]

I've seen speedo shops alter the magnetism in the speedo head.

[KQQL IT]

Same experience with mine.

[Steeveedee]

Back on the topic of speedo accuracy- I got pulled over for doing 66 in a 55 many years ago, in my '69 Camaro. I had the nerve to ask the CHP officer if his speedo was calibrated. Hey, I was just a kid. Of course his speedo was calibrated, duh.

[Fbks_Cruiser]

So I went for a drive today with the GPS and took some notes.

After driving 12.0 miles by the odometer the gps showed 11.4 miles traveled. So the odometer reads about 5% high which is close enough for me.

But the speedometer is a another story. At 20mph on the speedometer, gps shows 25, at 40mph on the speedometer, gps shows 47 and at 60mph on the speedometer the gps shows 68 mph.

So odometer reads slightly high and the speedometer reads quite a bit low. Looks like the speedometer is out of calibration.

[pjmoreland]

It sounds like part of your speedometer reading is offset a fixed amount of around 5-8 mph regardless of the speed. This type of offset can be adjusted by tightening or loosening the spiral spring in the speedometer. Loosening the spring makes the speedometer read higher. There is a bent steel piece inside the speedometer that can be scooted to adjust the spring. It takes a very small amount of movement to make a big change (a 1/16" shift of the tip of the metal piece will make a big change). You're basically doing the same thing as rotating the entire speedometer relative to the numbers on the lens by scooting the bent metal piece. Adjusting it is a matter of trial and error. I'd put a mark on there to indicate the original position.

Since your odometer isn't perfectly accurate, some of your speedometer error is due to slightly wrong speedo drive gears. I would focus on getting the Odometer perfectly accurate first before messing with the speedo spring.

[Fbks_Cruiser]

You may be on to something there. I noticed that I need to get up to 8-10 mph before the needle starts to move. Do you by chance know which way I would move the metal tab to loosen the spring?

[pjmoreland]

My speedometer had the exact same behavior where it wouldn't move off zero until I was going about 5mph. It was off by 5mph everywhere. My Odometer, however, was dead on. I did a 10 mile drive watching mile markers, and it was exactly right. The speedometer was off though, so I rotated that bent steel piece slightly to get the speedo right.

If you are looking at the back of the speedometer, then the bent metal piece needs to be rotated counterclockwise to increase the speedometer reading.

[Fbks_Cruiser]

Thanks. I hate pulling the instrument cluster but I’ll give that a shot.

[Fbks_Cruiser]

Well, I spent a couple of hours today pulling the instrument cluster again. I removed the speedometer from the cluster and connected it back on the speedometer cable so that I could test drive and adjust that little lever pjmoreland referenced. It took about 4 tries to figure out the correct direction to move the lever and how far. (Clockwise about 3/16”). I had to move it further than I expected. But, I am very pleased with the results. The speedometer now reads within 1 mph of the gps between 30 and 60 mph! It’s a little off at the low end but I’m okay with that.
Thanks pjmoreland!
After that success, I had to give her a CLR bath. Here are a couple pics.

['68OrangeSunshine]

I was working behind the Instrument Panel to clean up and reinsulate some adjacent wiring after my Cigar Lighter circuit burned up. [Put a whole new lighter socket and wires in, now with an inline 15 A fuse]. While the dash was off, I disconnected the Speedo cable at the SM465, and pulled the inner cable out of the sheath. As I slipped it back in, it was dusted liberally with graphite lube powder. After reassembly, I noticed my Odometer had ''travelled 2 miles'' sitting still. The lubing was otherwise successful, and the needle hardly jumps, and is within 2 - 3 MPH of the GPS indicated ground speed.

[pjmoreland]

I'm glad to hear it worked out. Well done. I guess I should have said clockwise when viewed from the front instead of counterclockwise when viewed from the back. Sorry about the confusion.