Speedometer/ odometer

[P-12 Tommy]

Hope everybody had a great Christmas. The only thing is it doen't look right on Christmas with it being 75 and no snow. Not that I'm complaining. I've got a question. I took the '41 for a run from Melbourne Beach to Malabar, Fl. Round trip 31 miles. Per GPS. What's really weird is the odometer was right on the money. But the speedo is still reading 15 MPH too slow. It (speedo) runs quiet and doesn't bounce around. I bought it as an NOS unit about a year and a half ago. Besides that, the Plymouth ran super, ran cool(175 to 185) and I was running with traffic at 55 to 60 MPH with the speedo reading 40 to 45 MPH with no problem.

Thanks, Tom

[P-12 Tommy]

I believe the odometer is more or less directly driven by the speedo cable, but the needle itself is more of an induction setup driven by a spinning magnet, or something like that.

Thus they can be in disagreement.

Thanks Shel, Appreciate that. I guess I still have to buy one of them there speedo reduction gear thingy.

Tom

[greg g]

It might be as simple as a bit of dust accumulated within the instrument causing the needle to drag against the spiing magnet. I think Dave Maxwell's site (linked off his members page) has a section showing the workings. His is of a p15 but the concept is the same. Ccheck it out maybe you can get the cluster out annd give it a gentle cleaning an lubrication.

[james curl]

The needle is controled by the tension on the hair spring, after oiling all the trunion points you will need a source of power that is 1000 RPM. With the speedometer out of the car and the hair spring exposed hook up a square drive,(peice of old speedometer cable) to the speedometer and drive it 1000 RPM which should indicate 60 MPH. If it doesn't you can adjust the tension on the hairspring by moving the movable anchor until 1000 RPM gives you 60 MPH. After oiling I would let it run at 1000 RPM for some time before adjusting because the oil might reduce the drag on the bearings and change the reading.

[P-12 Tommy]

Thanks to greg g and james curl for the responces. I'm gonna hang out and see what some of the other guys have to say.

Thanks, Tom

[martybose]

I think your speedo gearing is correct, as your measured distance was in agreement.

I may not be correct in that thinking:cool: Maybe we can get some guru/zen master input on that:o

You're absolutely right; if the odometer is reading correctly, the gearing is correct.

The speedo can be tricky; right now my odometer is correct, and the speedo is dead on at 70 MPH, but it reads about 6 MPH low at 30 MPH. I'll figure it out eventually.

Marty