Gas mileage calculator
The only accurate way to measure real economy is brim to brim: fill until the pump clicks off, reset the trip, drive normally, then fill to the click again. Reading a half tank introduces the error of where exactly the pump cut out, which on a 60 litre tank can be two litres in either direction. Enough to move an MPG figure by ten per cent.
Fuel economy is distance divided by fuel used. Three hundred miles on ten US gallons is 30 MPG, which is 36 imperial MPG or 7.84 litres per 100 km. Measure brim to brim over a full tank for a figure worth trusting.
How to measure gas mileage
The two gallons are the reason this page exists. A US gallon is 3.785 litres and an imperial gallon is 4.546, so the same car returns about 20% more MPG on paper in Britain than in America without burning a drop less fuel. Comparing a US and a UK figure directly is one of the most common apples-to-oranges errors in car buying. Litres per 100 km avoids the whole problem, and has the further advantage of being linear in cost: halving l/100km halves the fuel bill, whereas going from 30 to 40 MPG saves less than going from 20 to 30.
Questions
Divide the miles driven since the last fill by the gallons it took to refill. Measure over a full tank for accuracy.
An imperial gallon is 4.546 litres against a US gallon of 3.785. The same car scores about 20% higher in imperial.
It depends entirely on the vehicle. What matters more is your own trend: a sudden 10% drop usually means tyre pressure, a sensor or a dragging brake.
It is more useful for money, because it is linear: 8 to 6 l/100km saves exactly a quarter of the fuel, whereas MPG improvements shrink as the number rises.
Rarely exactly. Most trip computers read a few per cent optimistic. Brim-to-brim is the honest figure.