Dont know about the wiring but they are very accurate.
They calculate your MPG and mileage by measuring your speed and fuel flow, so fuel used/miles travelled=MPG and then the amount of fuel in your tank/MPG=Mileage left before empty.

Annoyingly since converting to EFI I cannot use mine as the fuel flow sensor went inline with one of the MFI injectors and as they all now run from a fuel rail I cannot plumb it in anywhere