Early 240 speedometer error

I've been recently working on sorting out the speedometer in my 1980 wagon. The car came with slightly smaller tires than the stock 185R14 tires and the speedometer needle originally bounced quite a bit while reading a decent amount under at 5-10mph on the highway.

Since then I've replaced the cable, which fixed the bouncing problem, and put 195/75R14s on it, which should be the same diameter as the original 185R14 tires. The car has a speedometer marked as r0.960 which should be correct for the larger diameter of the wagon tires.

The speedometer gear in the transmission is a 20 tooth black gear, which as far as I can tell is correct for a 3.91:1 rear axle. When I bought the car the seller said the rear end was 3.54:1, which should have an 18 tooth gear according to this thread, http://forums.turbobricks.com/showthread.php?t=272631. I am going to try seeing if the odometer is reading correctly today. There is a length of road near my house that seems to be 1 mile long according to google maps.

Fyi the sticker is on the axle drivers side facing to the rear.

122power said:

Fyi the sticker is on the axle drivers side facing to the rear.

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Yep, I have to go down there and check it after work today.

If it's not off more than about 3 or 4 mph. I'd keep rolling with it. My 93 wagon reads 5mph fast. Even with the stock size tires which for my wagon was the 185/70-14 tire. 185/70 and 195/70 are both good upgrade sizes which work well for the older wagons.

dl242gt said:

If it's not off more than about 3 or 4 mph. I'd keep rolling with it. My 93 wagon reads 5mph fast. Even with the stock size tires which for my wagon was the 185/70-14 tire. 185/70 and 195/70 are both good upgrade sizes which work well for the older wagons.

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My goal is to have it off by that much. 5mph I would be fine with, but it seems to be closer to 10 on the highway. Enough that I have to do the math to get the speed from engine RPM. Luckily I stuck an oem tach in it.

The thread you linked to refers to AW71 speedometer gears. They aren't the same as M46 gears.

An M46 car with a 3.54 rear end should use this blue 18 tooth gear.

https://vp-autoparts.com/en/artiklar/gear-speedometer-18-tooth-od-j-type.html

I should have a good used one. Let me know if you need it.

hiperfauto said:

The thread you linked to refers to AW71 speedometer gears. They aren't the same as M46 gears.

An M46 car with a 3.54 rear end should use this blue 18 tooth gear.

https://vp-autoparts.com/en/artiklar/gear-speedometer-18-tooth-od-j-type.html

I should have a good used one. Let me know if you need it.

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Would a 3.91 still use the 20 tooth gear on an M46?

Actually both 3.91 and 3.31 ratios use the black 20 tooth gear. The metal gear inside the OD has 7 teeth instead of 6 for a 3.31 though. The tail housing should have a 6 or 7 stamped into it near the rear seal to identify which gear is inside.

I would do what you said (test the trip meter for accurate distance) and use whichever gear gets you the closest to exact miles. If in doing so the speed reads slightly too high or low, so be it. That's how I would do it

hiperfauto said:

Actually both 3.91 and 3.31 ratios use the black 20 tooth gear. The metal gear inside the OD has 7 teeth instead of 6 for a 3.31 though. The tail housing should have a 6 or 7 stamped into it near the rear seal to identify which gear is inside.

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Interesting. Another thing that could cause a weird reading. Judging from the tool marks on the fasteners, the transmission has been out at least once before. So who knows if it is original to the car.

Checked the sticker on the axle tube, 3.91:1, and we've already got the right 20 tooth gear for that.

Took it out on the 1 mile stretch of road and the trip odometer ticked over to 1 mile just as I rolled to a stop at the end of the road. (Means my MPG math is right).

I took apart the cluster to check for any dirt or grime in there, nothing.

On my road test I got the speedometer reading 2mph slow at 27mph and 5mph slow at 60mph. Double checking both with the tachometer and a GPS speedometer on my phone strapped to the dash. All said a total error of about 8%.

I'd guess the old thing is just tired and in need of recalibration after 42 years. Time to live with it for now and take it down to a speedo shop someday.

You can always pull the needle off and put it back on slightly different. That may help make up for the different reading.

dl242gt said:

You can always pull the needle off and put it back on slightly different. That may help make up for the different reading.

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One thing that I did think about is that when I lift it off the little peg it drops and hangs downward. So there is some pre-load on the spring when it is sitting on the peg. I guess if I lessen that pre-load the spring by tweaking the needle resting position when off the peg it will be lighter by just a bit and start moving sooner. I'll try it and see.

Really I am not going to worry about it much right now.

Edit: There is a calibration mark where the needle is supposed to hang when not on the peg (thank you VDO for having those on everything). So I moved it a bit above that. I've also got another parts speedometer I can try monkey with parts from, maybe swap the magnet and see what happens. But that is a sedan speedometer with different odometer gearing and different calibration.

So changing the pre load on the needle as it sits on the peg just lets me change it from being 8% over all the time to reading fast at low speeds at reading accurately at high. I'd rather it just be off the same percentage across the board. Probably a magnet issue.