Here's something that bugs me with my '79, and it's been this way as long as I can remember. The speedometer reads about 10% slow, so when it says 50 mph I'm actually doing about 55 mph. Now, I know this is a simple gear change to recalibrate, but here's the weird part: the odometer reads about 3% FAST (the other way), so they are about 13% off from each other.

My understanding is the odometer is just geared off the speedometer and there is no adjustment, so how can these be off from each other? Were they this inaccurate from the factory?

Is there a way to correct the speedometer without throwing the odometer even further off?

Thanks!

 

It might be two different problems rather than just a simple gear change.
You might consider taking it to a shop that specializes in instrument and speedometer repair.
I see several listed every month in Hemming's Motor News.

 

Hard to say, but one would tend to assume someone rolled the ODO back at one time, somehow messing up the internal workings. The speedo needle is moved by magnetic force, the ODO by gears.

Josh

 

Thanks for your input. Odometer hasn't been rolled back, that I know for sure. My grandfather bought the truck new and only work it's had done was an engine swap, tranny rebuild, and transfer case change, all of which I did.

 

Just did a little reading on how a speedometer works. Since the odometer is gear driven, and off by only 2.8%, that is just a slight tire size difference from factory. The speedometer reading being off must have something to do with the speedometer head, either the magnet, spring, or coil. I'll leave it alone for now, since it is predictable at least, I just have to keep in mind that I'm going 10% faster than it says.