Speedo way off, odometer off a little

[Dibbons]

Installed some rallye gauges (150 mph speedo) in my '72 SSP 318 automatic project car that I purchased on ebay. The odometer is about 5% too high, but the speedo is way off (i.e. reads about 80 mph when doing 60 mph). How does one explain that? The 8 1/4 rear axle ratio is 2.76.


Then today on ebay I find a rallye 120 mph speedo, never saw one before. What year/model did this come on?

 

[kiwigtx]

You probably need to change the pinion drive in the transmission.
If the speedo reading is higher than the previous odometer, and nothing else has changed, there is little else you can do apart from re-calibration by way of changing your pinion gear as above.

 

[Dibbons]

The odometer that is 5% off is the same odometer that came with the wacko speedometer, I purchased all the gauges as a set. But like you said, I probably only have one option. Older speedometers seem to do odd things.

 

[idrivemopar]

Yep, double check which speedometer gear you have in your transmission. Easy to check, just measure your rear tire diameter and look it up on this chart with your gear ratio to see how many teeth the gear should be and verify what you have.

 

[Jcharger]

The speedo uses a hair spring that works against the magnetic torque created by the speedo head. I am told that sometimes this spring weakens, the speedo gets gummed up, or the magnets in the speedo weakens. If it is miscalibrated, you can get it calibrated at someplace like redline gauges. The odometer is mechanical and its reading is a directly related to the times the speedo cable rotates. If you want both to work, the only option is to get it calibrated or replace it. You will also need to change the pinion gear to have the odometer work correctly. If you want to just set the pinion gear to work with the miscalibrated speedo it can probably be made to read the correct speed, but you will want to make sure it does not drift with time. Something has happened to cause it to do what it is doing now.

I had a similar issue and decided to buy a replacement restoration one (I think from classic industries) which was expensive, but has worked well for me.