Speedometer error - cause?

[Bob H]

One of my deuces has a speedo that reads fast.
No GPS or pace car, I just know it won't do 60-65 mph at 2400 rpm on stock tires.

First I assumed that it had a speedo adapter for the 1100/20's,
so I swapped it with the one on my long bed with 1100R20's, as it reads slow with the bigger tires.

That made no difference.

Any ideas?????????????

 

[glcaines]

Most likely a faulty speedometer head.

 

[jaxsof]

Speedometers in North America are calibrated to read 60 MPH at 1000 rpm on the speedo cable. They are driven by magnets, and controlled with springs, the older analog ones anyway. We wont get into digital today. There is a tiny screw on it somewhere with which it can be calibrated. First, though you are going to need a 1000 rpm source that can be trusted. I dont have one that can be trusted, myself. Your local State Police can direct you to a facility that can calibrate and certify, if you need be police accuracy. From there you calibrate to your tires through the gear in the drivetrain adapter.

They do lose calibration, and most of ours are pushing 40+ years.

Hope this helps.

 

[Barrman]

The M35 came with 9.00-20 tires. The M34 came with 11.00-20 tires. All the major components were the same except for the tire size. So, two different tire sizes were accounted for by different speedometer reduction boxes on the transfer case. If you have 9.00-20 tires and your speedo is about 10-12% fast. Then, you probably have the reduction box for 11.00-20 tires on. I have a good 9.00-20 box and 11.00 tires on my truck. My speedometer is about 10-12% slow. Want to swap?