Gas Mileage Calculator: Work Out Your Real Economy
Trip meter and pump receipt in, real mpg out, with the metric equivalent and the cost per 100 miles alongside it.
The trip computer in most cars is optimistic, and the window sticker was measured in a laboratory. The only economy figure worth budgeting on is the one you measure yourself: distance driven divided by fuel it took to fill back up.
Measure it properly, once
The method is simple and the discipline matters more than the arithmetic.
Fill the tank until the pump clicks off, and do not top it up further
Reset the trip meter at the pump
Drive normally until the tank is at least half used
Fill again at the same pump, to the same first click, and note the litres or gallons added
Enter that distance and that volume above
One tank is a sample, not a verdict. A brimmed tank on a slope, or a pump that cuts off early, moves the answer by a few percent on its own.
Repeat over three tanks and average them before you trust the figure.
What the result tells you
The headline number is your observed economy. The second figure is the same result in the other unit system, which is useful if your car reports one and you think in the other.
The cost line turns economy into money: what 100 miles of driving costs at the pump price you entered. That is the figure to compare across cars, because a 5 mpg difference means very different money at $3 a gallon than at $5.
What a 40 mile daily commute costs at $3.40 a gallon
Observed economy
Per 100 miles
Per year, 250 commuting days
22 mpg
$15.45
$1,545
30 mpg
$11.33
$1,133
38 mpg
$8.95
$895
When your economy drops, read it as a symptom
A sudden fall of 10 percent or more, with the same route and the same driving, is diagnostic information. The usual causes are cheap and worth checking in order: under-inflated tyres, a clogged air filter, a dragging brake, or a failing oxygen sensor.
Tyre pressure is the one to check first because it is free and it moves economy measurably. The procedure is in how to check tire pressure.
Once you know your real economy, use it rather than the sticker figure everywhere the number matters. Price a trip or a commute with the fuel cost calculator, and check it against what other owners report on the model profile before you decide the car is at fault.
Frequently Asked Questions
How do I calculate my car's real MPG?
Fill the tank, reset the trip meter, drive until the tank is at least half used, then fill again and note the fuel added. Divide the distance by that volume. Enter both figures above and the calculator returns mpg, the metric equivalent, and the cost per 100 miles.
Why is my trip computer different from the calculated figure?
Trip computers estimate from injector pulse and are often optimistic by a few percent. The fill-to-fill measurement uses fuel you actually paid for, so treat it as the reference and use the computer only for trends.
How many tanks should I measure?
At least three, filled at the same pump to the same first click. Single-tank results carry too much error from how full the tank was at each fill to be worth acting on.
What does mpg convert to in litres per 100 km?
Divide 235.2 by the mpg figure. 30 mpg is 7.8 litres per 100 km and 40 mpg is 5.9. The relationship is reciprocal, so higher mpg and lower litres per 100 km both mean better economy.
My fuel economy suddenly dropped. What should I check?
Tyre pressure first, because it is free and it moves the number. Then the air filter, a dragging brake, and the oxygen sensor. Rule out cold weather and a change of route before assuming a mechanical fault.