I suppose re-purposing might apply to the tach, but the 'fuel' and temp gauges are pretty analogous to their original purpose .
The ZEVA driver converts the amount or power moving through the HV cable to a set of pulses the tachometer can understand. It also 'counts' the amount of total power used, and adds a bit of resistance to the fuel gauge, making it move down. When charging, it subtracts the resistance, making the fuel gauge go up. If you only give it a partial charge, (like at a public station) it only goes up the amount of charge it receives. Pretty clever, and it seems to me to be relatively accurate - I haven't had to reset it since I calibrated it a while back.