Gas Mileage Calculator

Measuring it properly

This calculator measures what your car actually returns, which is usually not what the manufacturer claims. The method is the tank-to-tank one: fill the tank completely, note the odometer, drive normally, then fill completely again and record both the distance covered and the fuel it took.

The critical detail is filling to the same point both times — to the first click of the pump nozzle, on level ground, at the same station if you can. The fuel you put in at the second fill-up is exactly what the drive consumed, so any inconsistency in how full "full" is goes straight into the answer. A single tank is also a small sample; averaging across three or four fill-ups gives a much more trustworthy figure.

If you want the cost of a specific journey rather than your car's economy, the Fuel Cost Calculator is the tool for that — it takes a known efficiency and a distance and returns a trip cost.

The units, and why they disagree

Three systems are in common use and they are not interchangeable.

MPG is distance per unit of fuel — higher is better. The trap is that a US gallon is 3.785 litres and an imperial gallon is 4.546, so imperial MPG figures are about 20% higher than US ones for the identical car. A 30 US MPG car is 36 imperial MPG. British and American figures quoted side by side are frequently compared without this being noticed.

L/100 km is fuel per unit of distance — lower is better. It is the standard across Europe and much of the world, and it is arguably the more sensible unit, for a reason worth understanding.

km/L is used in India, Japan and parts of Latin America, and works like MPG.

Why L/100 km is the better measure

MPG is a reciprocal, and reciprocals distort comparisons. Consider improving a car from 10 to 15 MPG, and another from 30 to 35 MPG. The second looks like the same 5 MPG gain, but over 10,000 miles the first saves 333 gallons and the second saves 48. The low-efficiency improvement is worth seven times as much.

In L/100 km this is immediately obvious, because the unit is linear in fuel consumed. This is sometimes called the "MPG illusion", and it has real consequences — it makes replacing an inefficient vehicle look less valuable than it is, and upgrading an already-efficient one look better than it is.

Conversions

Why your figure is below the official one

Official economy figures come from standardised laboratory cycles, not from roads. Real driving differs in almost every respect that matters: short trips where the engine never reaches operating temperature can cut economy by a third or more; cold weather thickens oil, increases idling and runs the heater; air conditioning costs a few percent; roof boxes and bike racks add substantial aerodynamic drag at speed.

Under-inflated tyres are the most commonly overlooked factor and one of the easiest to fix. Aggressive acceleration and braking is the largest behavioural one — smooth driving genuinely returns 10 to 20% better economy than the same car driven hard, which is a bigger effect than most fuel-saving gadgets claim and it costs nothing.

Speed matters more than people expect above about 90 km/h, because aerodynamic drag rises with the square of speed. The difference between 100 and 120 km/h on a motorway is typically 10 to 15% in consumption.

How do I measure my real fuel economy?

Fill the tank completely and note the odometer. Drive normally. Fill completely again, and record both the distance since the last fill and the fuel it took. The fuel added is exactly what the drive used. Fill to the same point each time — the first click of the nozzle, on level ground — because any variation in how full "full" is goes straight into the result. Average three or four tanks for a figure you can trust.

Why is imperial MPG higher than US MPG?

Because the gallons differ. A US gallon is 3.785 litres; an imperial gallon is 4.546 — about 20% larger. So the same car returns roughly 20% more miles per imperial gallon than per US gallon. A 30 US MPG car is 36 imperial MPG. This catches people constantly when comparing British and American figures, which are often quoted side by side with no indication of which gallon is meant.

Is L/100 km better than MPG?

As a measure of fuel consumed, yes. MPG is a reciprocal, which distorts comparisons: improving from 10 to 15 MPG saves far more fuel than improving from 30 to 35 MPG, despite both looking like a 5 MPG gain. Over 10,000 miles the first saves 333 gallons and the second 48. L/100 km is linear in fuel used, so the difference is immediately visible. This is known as the MPG illusion.

Why is my mileage worse than the official rating?

Official figures come from standardised laboratory test cycles rather than real roads. Short trips where the engine never warms up can cut economy by a third. Cold weather, air conditioning, roof racks, under-inflated tyres and heavy traffic all reduce it further. The largest single factor you control is driving style — smooth acceleration and anticipation typically returns 10 to 20% better than the same car driven aggressively.

Does driving slower actually save fuel?

Above about 90 km/h, considerably. Aerodynamic drag rises with the square of speed, so the power needed to overcome it rises steeply. Dropping from 120 to 100 km/h typically cuts consumption 10 to 15%. Below that speed the relationship is weaker and very slow driving in a high gear can actually be less efficient. Steady speed matters as much as the number — repeated acceleration and braking wastes far more than cruising a little faster.