Multiple vehicles, each with its own units
Add every vehicle you drive and give each one its own distance and volume settings. A Canadian driver filling up in the United States can run one vehicle in kilometres and litres and another in miles and US gallons, with Imperial gallons available where that is what the pump means. Fuel economy is calculated in the units that vehicle uses, so the number matches what you would work out on paper.
Miles or kilometres.
US gallons, Imperial gallons, or litres.
MileMatrix