Speedometer Reads low over 60 mph only

[Still Bill]

1998 Ranger 4-cylinder automatic. No check engine lights. Running great.

Instrument cluster was replaced when we got vehicle last year because speedo did not work at all. Since then, had transmission rebuilt, and had intermittent problems traced to bad GEM. Those problems included speedo not working.

Replaced GEM with used unit. No more problems.

Except - my son noticed that his speedometer does not go over 65mph. We recently compared with me driving in front of him in different vehicle and comparing speeds on phone.

At 35, 40, 45, 50, 55 mph speedo is exactly correct. When his speedo shows 60, he is actually going 65. When it shows 65, he is actually going 70. His speedo won't show over 65, even when he is doing 80.

I realize GEM is for different vehicle, maybe one with standard trans, or different size wheels, but still it is accurate under 60, so I don't think that would make a difference.

Also, he said speedo did not go over 65 BEFORE we changed GEM.

Think it may be the actual speedometer is bad? Unit shows only 78,000 miles.

 

[RonD]

Could be something in the gauge is preventing needle from progressing farther without more than normal force.

I wouldn't think this would be a VSS or GEM issue

You can test and calibrate electric speedometer with an AA Battery
Test seen here: https://www.ranger-forums.com/general-technical-electrical-18/calibrating-speedometer-needles-134925/
Calibration is at 72MPH so you should see the problem if it tops out at 65mph

There is also a video on doing this: www.youtube.com/watch?v=WVz6ilGiTBY

I think electric speedometer works the same as cable driven, electric motor speed increases with VSS voltage increase, turning the magnet faster, which turns the "cup" attached to the needle more, a hair spring keeps the cup from jumping around, holds it against faster spinning magnet
Diagram here: https://www.explainthatstuff.com/how-speedometer-works.html


It is possible, but a long shot, that VSS sensor is maxing out, VSS is a Variable Reluctance sensor, it generates its own power, AC Volts.
In the rear axle is a tone wheel, with teeth like a gear, each time a tooth passes by the VSS it generates an AC voltage pulse.
The Frequency of these pulses are what set the speed but it is a Voltage that also goes up with speed.
It might be possible for the VSS not to go above a certain frequency/voltage, although I have never heard of that happening

 

[Still Bill]

Thanks for input. I just ordered a used instrument cluster from ebay for $70. As long as I will have the unit out to test, I might as well try a swap. If it turns out to be something else, I'll sell one.