ReckonBoxMeasuring what your car actually gets
Gas mileage is the distance a car travels per unit of fuel, found by dividing the miles driven by the gallons used. Drive 300 miles, burn 10 gallons, and that is 30 miles per gallon. One division, no guesswork.
The reliable way to measure it beats trusting the dashboard readout. Fill the tank, reset the trip meter, drive normally for a tank or two, then refill. The pump tells you the gallons, the trip meter tells you the miles, and the ratio is your real economy. This tool then shows that number in all four units at once, because a figure that reads as 30 in the US reads as 36 in Britain and as 7.84 the metric way, and mixing them up is easy.
Four ways to say the same thing
US MPG, Imperial MPG, kilometers per liter, and liters per 100 km all describe one car's fuel use, but they do not read the same. Two of them go up when the car is more efficient, and one goes down.
Liters per 100 km is the odd one out: it measures fuel consumed for a fixed distance, so a lower number is better, the reverse of MPG. And the two gallons are different sizes, so the same car scores higher in Imperial MPG than US MPG. Getting these straight matters when a US spec sheet and a European one sit side by side.
| 30 US MPG equals | Value |
|---|---|
| Imperial MPG | 36.03 |
| km per liter | 12.75 |
| L per 100 km | 7.84 |
The conversions worth knowing
Liters per 100 km is 235.215 divided by the US MPG, so 30 MPG is 7.84 L per 100 km. Imperial MPG is US MPG times 1.20095, because a UK gallon is 4.546 liters against the US gallon's 3.785, so 30 US MPG is 36.03 Imperial. Kilometers per liter is US MPG times 0.425144. The tool runs all three off the single MPG figure so the numbers stay consistent with each other.
What this does not cover
This measures the economy of the fuel you actually burned over the miles you actually drove, which is the honest figure. It does not project an annual fuel bill or compare two cars on cost, since that needs the fuel price and your mileage, which the fuel cost calculator handles. It also assumes a full-to-full measurement; a single partial fill-up gives a noisy number, and one highway trip or one week of short city hops can swing a single tank well away from your true average.
Measure across three or four tanks and the figure settles into something you can actually plan around.
Frequently asked questions
How do I calculate gas mileage? Divide the miles driven by the gallons used. If you drove 300 miles and it took 10 gallons to refill, that is 30 miles per gallon. The cleanest way is to fill the tank, reset the trip meter, drive, and refill: the pump shows the gallons and the trip meter shows the miles, so one division gives your real MPG.
What is a good MPG? It depends on the vehicle class, but a modern compact or midsize gas car often lands between 25 and 35 MPG combined, while trucks and large SUVs run 15 to 22 and hybrids reach 40 to 55. Your own measured economy is the honest figure to compare against the EPA rating on the window sticker.
How do I convert MPG to L per 100 km? Divide 235.215 by the MPG. So 30 US MPG is 235.215 divided by 30, about 7.84 L per 100 km. The two scales run opposite ways: a higher MPG is better, but a lower L per 100 km is better, since it measures fuel used rather than distance covered. This tool shows both so a US and a metric figure line up.
Why is US MPG different from Imperial MPG? Because the gallons are different sizes. A US gallon is 3.785 liters and an Imperial gallon is 4.546 liters, so the same car reads higher in Imperial MPG. Multiply US MPG by 1.20095 to get Imperial: 30 US MPG is about 36.03 Imperial. Comparing a US and a UK figure without converting overstates the difference by about 20%.
Why is my measured MPG lower than the sticker? Because the EPA rating comes from controlled lab cycles, and real driving adds cold starts, short trips, high speeds, cargo, and terrain that all cut economy. A gap of 10 to 20% below the sticker is common in mixed driving. Measuring over several tanks, not one, smooths out the effect of a single highway run or a week of city errands.