Strange speedometer behavior

[Pete]

I wonder if anyone else has seen this.

I have the original speedometers in both my 1939 Plymouth touring sedan and my 1938 Dodge Brothers pickup. Both gauges have been professionally rebuilt and calibrated, although by two different shops. I still run 600/16 wheels on both. I have bias ply on the truck and radials on the sedan.

When I drive checking speed with a GPS, both speedometers are accurate at higher speeds -- 40 to 50 MPH. However, they are way off at lower speeds. When the GPS indicates 30 MPH the speedometer says 17 MPH. The variance gets smaller as the speed increases to around 40 MPH. After 50 MPH of so the inaccuracy goes the other way. I've tested them with a number of different GPS. I have a drawer full as I used to work for a company that makes them.

I spoke with tech support at one of the companies that rebuilt one of my speedometers. He said an adapter would not fix this issue as they only correct for a percentage. For example, if the gauge is off by 1 MPH at 7 MPH, it would be off by 10 MPH at 70 MPH (or something close to that). Does that make sense?

So, is anyone seeing this behavior in their vehicle? If so, were you able to fix it?

Pete

 

[Plymouthy Adams]

have you checked your cable hand over hand for kinks or pulled the cable to inspect for wear pattern indicating bad internal housing and drag...when was the last time you lubed the cable/housing.

[kencombs]

I haven't looked  inside a speedometer of that age, but most i have seen are magnetic drive.  If the clearance between the driven and driving elements is wrong that can happen.

 

I think a bench check by another shop may be in order.

 

One question:  Is the odometer accurate? 

[Pete]

Hi PA,

The cable is good -- no kinks or binding. I cleaned an lubed it last year.

Pete

 

[Pete]

Hi Ken,

I need to do a check on the odometer. Hopefully I can get to that soon.

Pete

 

[Plymouthy Adams]

you are stuck in a time warp......hot bed for alien activity in your area...first sign you may want to move.....?

[Pete]

So I need to wear an aluminum foil hat?

[Dodgeb4ya]

The magnets can get weak causing calibration issues. Another speedo shop inspection required.

[Plymouthy Adams]

2 hours ago, Pete said:

So I need to wear an aluminum foil hat?

Tom Terrific and Manfred his wonder dog had it right oh so many decades ago...