WHen you calibrate, UG does not care what units you use. If you say there were 5 gallons/liters/quarts what ever, used, then UG will consider the amount of fuel used to be 5.0 These could be Liters, gallons, guarts, imperial gallons, etc. So in effect you are calibrating the unit as you would expect, but you are also effectively setting the units. Most users think in terms of US gallons, but UG does not care and if you calibrate using imperial gallons....great.