MPG Calculator
Fill the tank, drive, fill it again, and put the two numbers in here. This is the figure your car actually returns, rather than the one on the brochure.
Fuel economy
28.1mpg
- Litres per 100km
- 8.38
- Miles per US gallon
- 28.1
- Miles per imperial gallon
- 33.7
- Kilometres per litre
- 11.93
- Measured from brim to brim: fill the tank, reset the trip meter, drive, then fill it again and use the litres the pump put in. A part fill guesses at the starting level.
- One tank is one sample. Averaging three or four covers the difference between a week of town driving and a run down the motorway.
- The dashboard figure is usually optimistic by a few per cent. This one is measured from what you actually paid for.
About measuring fuel economy
Every published economy figure comes from a standardised test. That makes them comparable with each other and not much use for predicting your own fuel bill, because the test does not include your commute, your right foot or your roof bars.
The measurement that does mean something is brim to brim. Fill the tank to the cut-off, note the odometer or reset the trip meter, drive normally, then fill to the cut-off again. The litres the pump put in are exactly the litres the journey used, and the distance is on the trip meter. No estimation is involved anywhere.
A part fill will not do it, because you would be guessing at where the level started. The same goes for the dashboard readout, which is calculated from injector timing and is usually optimistic by a few per cent.
One tank is one sample, and a sample of one covers whatever happened to be in that tank. Averaging three or four tanks across different kinds of driving gives a figure you can plan a year of fuel around.
What it works out
- Miles per gallon, US and imperial
- Litres per 100km and kilometres per litre
- From a brim-to-brim fill and a trip meter
- Any vehicle — car, van, motorcycle or truck
The formula
MPG = Distance ÷ Fuel used L/100km = Fuel used × 100 ÷ Distance
Distance divided by fuel gives miles per gallon. 320 miles on 11.4 gallons is 28.1 mpg.
The metric form divides the other way round and scales to a hundred kilometres, because litres per 100km measures fuel per distance rather than distance per fuel. The same tank is 8.38 L/100km.
Both describe the same tank of fuel. Which one is more useful depends on the question: mpg answers "how far will this take me", and L/100km answers "what will this journey cost".
The accuracy of the whole thing rests on both fills stopping at the same point. Pump cut-offs are consistent enough at the same pump, so filling at the same station twice removes most of the variation.
- Distance
- From the trip meter, reset when you filled up the first time.
- Fuel used
- What the pump delivered on the second fill — not the tank capacity.
A worked example
320 miles covered on a tank that took 11.4 US gallons to refill.
That works out to 28.1 mpg.
- Litres per 100km
- 8.38
- Miles per US gallon
- 28.1
- Miles per imperial gallon
- 33.7
- Kilometres per litre
- 11.93
Questions
How do I calculate MPG?
Divide the miles covered by the gallons it took to refill. 320 miles on 11.4 gallons is 28.1 mpg. For litres per 100km, multiply the litres by 100 and divide by the kilometres.
What does brim to brim mean?
Filling to the pump cut-off at the start and at the end, so the fuel the pump delivers the second time is exactly what the journey used. It is the only method that involves no estimate of the starting level.
Why is my dashboard MPG different?
The dashboard calculates from injector opening times rather than measuring fuel, and it is usually optimistic — commonly by three to five per cent, sometimes more. The pump receipt is the honest figure.
How many tanks should I measure?
Three or four, across different kinds of driving. A single tank tells you about that tank, and a week of short cold trips can read twenty per cent worse than a week with a motorway run in it.
Why is my economy worse in winter?
Several reasons at once: the engine spends longer cold and runs rich while it warms up, winter fuel blends carry slightly less energy, tyres are harder work in the cold, and heating and lights add load. A drop of ten to twenty per cent is normal.
Does a full tank of fuel hurt economy?
Barely. A full 50 litre tank weighs about 37kg, which is under three per cent of a typical car — less than a passenger. It is not a reason to run the tank low, which is harder on the fuel pump.
Is it worth driving to a cheaper petrol station?
Work it out rather than guessing. Saving five cents a gallon on a 12 gallon fill is 60 cents; a ten mile round trip in a 30 mpg car burns a third of a gallon, which costs more than that at any realistic price.
What is a good MPG figure?
It depends on the vehicle, so compare against the same model rather than an average. What matters more is the trend: a sudden drop of ten per cent with no change in your driving usually means tyre pressures, a dragging brake, or a sensor fault.