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Speedometer reads slow

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Secret Chimp

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Hey, first real post here so forgive me if this is in the wrong forum.

I just bought at 67 Coronet wagon - the speedometer and odometer work, but the speedometer reads slow. It doesn't jump around, it just reads 5-7mph slow around town and more than 10-15 mph slow on the highway. What can I check or adjust to see what's wrong? Could some part of the speedometer head/cable connection be too tight?
 
Secret Chimp said:
Hey, first real post here so forgive me if this is in the wrong forum.

I just bought at 67 Coronet wagon - the speedometer and odometer work, but the speedometer reads slow. It doesn't jump around, it just reads 5-7mph slow around town and more than 10-15 mph slow on the highway. What can I check or adjust to see what's wrong? Could some part of the speedometer head/cable connection be too tight?
Click to expand...

The speedo is driven off a gear in the transmission. If you get under your car you will see where the cable goes inside the trans back by the tailshaft. Depending on tire size and rear end ration the proper spline gear is installed on the end of the cable inside the trans. Year One used to have a chart in their catalog for tire size and gear ratio to select the proper color gear. If your car is running taller than stock tires or someone changed out the rear to a different gear you could have the wrong gear on the cable. It is a simple fix just be sure to index the piece that goes into the trans to the proper notch when you reinstall after changing the gear.
 
It's running stock size tires. The odometer turns over at the right rate despite the speedo running under, though (I'm assuming the speedo and odo use the same cable?)
 
If the mileage is accurate the speedo should be right. How are you checking mileage. You would probably need at least a 5 mile course to check the accuracy. Get someone in a newer all stock car to pace you at 50 and check your reading. If you know what you are off at 50 mph you can easily figure your % and live with it . The speedo and odo are driven by one cable and are connected in the speedo head.
 
I have found the best way to check mph is to go for a ride with my GPS on the dash. This will give you a very accurate reading. When I went through this, every one tooth gear change translated into a 1-2 mph change on the speedo.
 
I was wrong, I used the odometer on my GPS against the car's and it was about .3 miles off over one mile. With stock tires I guess the original gear could be worn down and effectively have a slower ratio? Should I get a gearbox or just a new stock gear and cable?
 
You don't need to drive for a mile to compare. What I do is get going 60 mph according to the gps, check what your speedometer reading is and write that # down. You will then have to pull your gear out to see what you have for a tooth count. Do this and measure the diameter of your rear tires. Do you know the rear end ratio? Post your results and we can get close on picking a new gear.
 
I'm assuming you checked your cable to make sure it is engaged all the way into the trans and dash first. You can disconnect the cable from the trans and spin thecable to see if the speedo jumps up. Otherwise I would think you have the wrong gear in the trans. I can't remember if I spun the cable with a drill or by hand.
 
The speedo and odometer run off the same cable but the speedo head is driven by an eddy current coupling (no physical contact) off the gear train. And it is possible for the odometer to be correct but the speedo to not read properly. There is a clock spring on the indicator needle shaft and that can be adjusted to fine tune the reading. Basically more tension on the spring will make the speedo read slower.
 
If you haven't fixed this yet, I have about 4 different gears in my drawer. Couldn't figure out the speed on my 66 Charger so I just kept going trial and error until I found the right one. They weren't that expensive and 30 seconds to change. I have the right one now so no need for the others if you want to try them. No charge. Don't recall if you can see my email address or not.
 
I've compared my GPS odometer against the car's over 100 indicated miles of driving and I've actually driven about 120, 60 mph is about 50mph indicated, so it looks like I'm about 3-4 teeth over what I should have in there. Phantom, I appreciate it & I'll take you up on those gears, send me a message so I can get my info to you.
 
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