mismatch between speedo and odometer

[str12-340]

I'm not sure if this is where this belongs, but here goes. I built a 70 Dart with a 318 and an A500 4 speed overdrive automatic (RH42). The speedometer was the best of 4 that I had as tested for me by someone who does dash gauge restoration. The speedo works fine - steady without fluctuation and the odometer clicks off the tenths and the whole unit seems to work fine. I have a GPS in my car that shows current MPH. It seems to be very accurate when you are going straight and steady, so I used it to figure out calibration of the speedometer and for once it seems to be dead on with the speedo gear now installed. I'm on a looong trip and have had some opportunities to check my odometer against 3 interstate measured 10 mile marked sections. In all cases the odometer records 9.0 miles for every 10 miles traveled. Has anyone ever experienced this??? It's easy enough to do the correction for mileage and distance traveled, but this is just weird... Any Ideas????

 

[Dana67Dart]

I believe there are adjustments in the speedo to fix that BUT I would not try to do it myself.

A speedo shop would be the best bet.

 

[GTX JOHN]

I never have observed an adjustment in the speedometer.

 

[67Dart273]

The speedo is driven by a magnetic coupling, and is subject to wear, to whatever gummed up lube is in there. Speedo might even be affected by loss of magnetism. The odo is gear drive connected to the cable. It can be incorrect because of the gearing, but it cannot slip like the speedo does

 

[Dana67Dart]

I never have observed an adjustment in the speedometer

Adjust the clock spring pressure changes the speedometer needle position.

As 67dart273 noted the odometer is gear driven.

Speedo shop would spin the head the correct rpm for say 60mph (1 mile per minute) the odometer would tick off 1 mile in 60 seconds. Then they adjust the spring to set the needle for the correct speed of 60mph.

Then the odometer and speedometer would be correct for a particular input rpm.

Your rear gear and tire size and speedo gear combo sets the closest setting to the correct vehicle speed.

Here is a head where the spring adjuster is not being held in place properly. The adjuster is pointing almost streight up.

A speedo shop could probably fix it.

And bouncing needles....

If the red box part ( which spins at speedo gear speed by the speedo cable) makes any contact with the green arrow part which is attached to the needle, the needle will bounce

 

[cudamark]

You need to select the speedo gear based on the odometer reading. Then adjust the speedometer needle tension spring for the correct MPH reading....or just live with the odometer being off a bit. Having a bit less on the odometer than it should wouldn't be the end of the world!