"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. "

so i guess it would be safe to say the guage is an aftermarket unisource guage?
i'd be willing to bet thats part of the problem
guages and senders now have to match certain specs to work together properly
if your mixing aftermarket and oem parts together this might not be such a good thing.