Beede 5" 120mph Speedometer reads high

10 years ago I bought Beede 5" gauges for my '53 Chevy truck from Haneline (I just now installed them). The speedometer is electronic and has 8 dip switches (today they have 12). The pulse generator is supposed to pulse at 8,000 per mile. The installation instructions include a chart for "calibrating" the speedo. The chart has an error range of 30mph to 78mph indicated when traveling 60mph. At 30mph the speedo showed 90mph. With all the switches in the on position the speedo indicates 120mph at 60mph. I've called Haneline but I'm not sure if the current staff is familiar with the "old" stuff (they haven't provided a solution yet). I have found a source that can make a 50% reducer for about $70 so that may be the way I go BUT I sure would like to get this thing working correctly. I haven't dug into the drive gears for the speedo yet, but I don't think changing them will have a 300% effect on the error. Anyone ever have a similar issue?

Thanks...