"replaced sender twice and gauge once".
Again we are faced with the possibility that the gauge in the instrument panel might not be correct.
All gauge system components, be they analog or digital, require some kind of "voltage regulating" component as part of the circuit in order to be able to display a CONSISTENT value over a range of voltage fluctuations (fluctuations which are normal in an automotive setting). Vehicles once had a separate "instrument panel constant voltage regulator" which distributed a regulated, "lower than battery" voltage supply to analog gauges in the instrument panel. The regulated voltage was usually at about 8VDC for 12 V systems and 4VDC for 6V systems. This was to assure that gauge readings would not fluctuate with normal drops (or peaks) in general system voltage.
Today's automotive world uses a similar system but to a much greater degree and the regulated voltage is often called "reference voltage" but the aim is the same.........keep the playing field level despite fluctuations of system voltage level.

Back in the day when ALL gauges were analog type, you could buy individual, "stand alone" gauges. These gauges were often available with their own built in voltage regulator circuit so that the gauge would operate properly when added to an application where no instrument panel regulator was present, or as a "work around" in cases where the OEM dash regulator was defective (and costly). Aftermarket gauges could also be obtained that DID NOT have a built in voltage regulator for use in applications where an instrument panel, constant voltage regulator was already in place.

I would wonder if perhaps somewhere along the way the gauge was replaced with with a non regulated dash gauge.
It might also the possible that if the Hyster was built having a constant voltage regulator for analog instruments......it has failed and now supplying a higher output voltage than it is supposed to resulting in the gauge showing a higher value. Typically when those regulators would fail it would be a "no output" failure but I supposed they could "short to full voltage".
I will admit that I am not familiar with that 1985 Hyster but many years ago we had a similar situation on an even older Hyster where the temperature gauge dash unit was replaced with a "looks like it" gauge from a local parts house and that gauge also displayed an overheat value at normal operating temperature.
It was quite humorous that the boss had carped about the cost of a gauge from Hyster so the cheapo gauge was purchased. When the boss was informed about the new gauge reading too high........he stupidly remarked "at least it's reading something now". The concept of why there are gauges clearly shot far over his head.