Fuel consumption calculator

Why your real MPG differs from the manufacturer’s figure

Manufacturer figures come from standardised lab cycles. Your number will differ, and the gap is usually explainable:

Short trips. A cold engine runs rich for the first few miles. A car used only for short journeys can sit 20-30% below its rated figure without anything being wrong.

Speed. Air resistance rises with the square of speed. The stretch between 70 and 80 mph costs far more fuel than the same 10 mph earlier in the range.

Load and drag. A roof box, a full boot, or roof bars left on all year are a permanent penalty at motorway speed.

Maintenance state. Underinflated tyres, a clogged air filter and tired plugs each take a few percent, and they add up quietly.

This is why one tank tells you little. Three or four consecutive tank-to-tank measurements give you a baseline; after that, a sudden drop is a mechanical signal, not bad luck.

The tank-to-tank method

The most reliable measurement: fill the tank COMPLETELY, note the mileage, and at the next COMPLETE fill write down the fuel you bought and the distance covered. mpg = miles ÷ gallons (or L/100km = litres ÷ km × 100). The dashboard average uses its own window and can be optimistic; tank-to-tank measures what you actually burned.

Reading the result

The result uses the units you entered. Enter miles and US gallons and the figure is US mpg; enter miles and litres and it is miles per litre, not mpg. If you buy fuel in litres, convert first: divide litres by 3.785 for US gallons, or by 4.546 for imperial gallons. An imperial gallon is about 20% larger than a US one, so the same car reads higher on the imperial scale.

Compare against YOUR OWN baseline, not the brochure. A single tank is noisy (AC, route, load); the average of 3-4 fills gives a true reading. A lasting unexplained rise in consumption is usually a maintenance signal (filters, tyre pressure, plugs) — see how to improve fuel economy and how to track car maintenance.

This number is only ONE part of what your car really costs — for the rest: cost per mile calculator and the true cost per mile.

FAQ

Does this calculator use US or imperial gallons?

It uses whatever you enter. The maths is a plain ratio, so the units of the result follow the units of the input. Enter miles and US gallons and you get US mpg; enter miles and litres and you get miles per litre, which is not mpg. An imperial gallon is 4.546 litres against the US gallon’s 3.785 — about 20% larger — so the same car reads higher on the imperial scale.

How many tanks do I need before the number means anything?

One tank is an estimate. Three or four consecutive tank-to-tank measurements give a usable baseline; once you have that, a sudden drop is worth investigating mechanically rather than writing off as a bad week.

Why is my real MPG lower than the manufacturer’s figure?

Manufacturer figures come from standardised lab cycles. Short trips, higher speeds, extra load and deferred maintenance each cost a few percent, and together they explain most of the gap.

Why does the dashboard show a different figure?

The car computes over its own window from engine data; tank-to-tank measures the fuel you actually bought. The difference is normal — trust tank-to-tank.

How big is the city vs highway gap?

Significant — city driving can use 1.5× the fuel of highway driving in the same car. Track the average of your own mix.

How many fills until it's reliable?

The average of 3-4 complete fills removes most of the single-measurement noise.

Does the calculator store my data?

No. It runs entirely in your browser; nothing you enter is sent to a server.