The original version REPLACES the clock assy internal to the inst cluster. Thus limited to BMW bikes only.
The last version used a separate mounted display unit. I have the later one on my '95LT.
Dave has now since quit making the units altogether. He has even let his internet domain name expire, someone else now has it.
The newer version has added 2 temp sensors, outside air and engine temp. The newer version was also designed to be installed on other brand bikes, not just BMW.
The theory of operation is that it measures the fuel used based upon the fuel injector pulses and a known value of fuel used per injector pulse.
It then takes the value of fuel used and subtracts it from value of fuel available in a full tank, and calculates close estimated # of miles/km left to ride before refueling.