70 XR7 speedometer calibration/repair

After 8 years of being in bodyshop/mechanic/interior jail, I finally have my car back and have a number of issues I still need to fix. I’m going to start with the speedometer. Currently when the speedometer reads 42 to 44mpg actual speed is about 70 to 72mph. I checked to make sure the speedometer cable gear at the transmission was the correct number of teeth for the tire size and gear ratio. I also checked for a reduction gear inline with the speedometer cable and didn’t see one. The lower cable comes out of the tranny and goes to the distributor modulation sensor. The upper speedometer cable goes from the distributor modulation sensor to the speedometer. The speedometer needle moves smoothly up and down with speed changes and is steady (does not bounce). Typically if the cables were dry or stripped I would think I would see the needle bouncing, but it doesn’t. It just seems like it is out of calibration. Is there a service manual for these online or a procedure for calibrating them or someone I can send it to for calibration? Any other ideas why the speedometer would be consistantly reading 26 to 30mph lower than actual speed?

Is your cat a Calif one?. It would have emissions tank & speedo devices inline. Since we don’t know the engine, trans & rear end ratio, this is a pure guess.
You reference a “reduction gear” . If your Cat had higher ratio gears it would have gotten a device inline to the speedo cable. Actually it goes into the trans & then the cable goes into it. Some thing like this - https://secure.cougarpartscatalog.com/c8zz-17294-a.html?attribs=87

If it does NOT have tall gears it could be the device inline that is bad. I usually will remove the original “Calif” speedo setup & replace with a single cable desiginated for the Cat w/ out emissions.

4 speed & automatic cables are different due to 4 speeds cable connecting on the Pass side of the trans and auto connect on the driver side.

Hope this helps!

My '70 speedo went south in just this way. Replaced it with another from WCCC.

351C-4V, FMX trans with 19 tooth speedo gear, 3.00 posi, 245/60r15 tires. I am aware of the reducer for the higher rear gears, so that is why I said I checked to make sure the shop or a previous owner hadn’t installed one. I was kinda hoping they had, because it would have been an easy fix to remove it. WCCC is out of stock of grade A XR7 speedometers. Hoping there is a way to pull mine and fix it. Not a California car.

I had really good luck with these guys fixing things up for us :


They did the GM gauges in the Jeep as well as fixing the speedo in a Nissan pickup that a friend had and were even able to explain to him why he had an issue and fix it so it wouldn’t happen again (though it lasted 20 years without issues).

Just one or two guys in a converted garage but it is grade A work. They even stayed open late on a Saturday so I could drop off the bits rather than shipping them since it’s only a couple of hours from here.

Thanks for the link. I will check them out.

I sent them an email explaining what is going on. We’ll see what response I get from them.

Can you bypass the modulation sensor to eliminate that as an issue?

I sure am no expert, but it sounds to me as if the needle base line is off, laying at the zero while in fact being already rotated as it should be at… 28 mph?? In such case, I think a symptom would be that the needle would not begin to climb immediatly when the car starts moving but only after it has reached a certain pace (28 mph ??). That would be consistent with your observation that your speedo is reading 26-30 mph less than actual speed, at all speeds.

That is along the lines of what I was thinking. Either the needle has slipped, the magnet that drives the needle has gotten weak with age and can’t fight the clock spring or pivot point is dirty so resisting the pull of the magnet. if it were dirty I would think it would be sticking though and not moving smooth like it does. if it were a weak magnet then I would think the speed variation would increase as speed increased. I’m thinking the needle has slipped so the start point is off. So now the question is how to get the needles start point back where it belongs. The shop listed above wants over $200 to fix it.

It’s been a while. I was able to get a replacement speedometer from WCCC, but hadn’t been able to test it until now. The new speedometer has sort of the opposite problem of the original. It starts out at 0 and is close at first but gets exponentially faster than actual speed. By the time I am going 40mph the guage is reading close to 80mph. I had my digital speedometer as a check and when I set the digital to Kph instead of Mph it almost tracks as if this speedometer came out of a European car. Getting frustrated with my guages.

The fuel guage doesn’t work right either. It moves from slightly below empty up to the empty line when I turn the ignition on, but never goes higher than that. I’ve replaced the fuel guage and the sending unit. I’m suspecting the fuel tank may not be well grounded. I also haven’t seen the low fuel light come on yet either, but that is difficult to test without running the tank dry.

I’ve been picking away at it and have all the other guages and lights working now. I found the tachometer was wired to the wrong side of the coil so it works now that it is connected to the correct side of the coil.