Mileage Calculator

The three ways to measure fuel economy

"Good mileage" means different things in different countries, and the unit you use quietly flips the maths. There are three common measures, and this calculator handles all of them:

Notice that MPG and km/L are "distance per fuel" (bigger is better), while L/100km is "fuel per distance" (smaller is better). They are inverses of each other, which is the source of endless confusion when people compare cars across regions. Enter your distance and fuel used, pick a unit, and the tool computes efficiency instantly — and crucially, it converts everything to an MPG baseline so it can compare any car against any other regardless of the unit you typed.

A worked example

Suppose you drove 300 miles on 10 gallons. In MPG mode the calculator computes 300 ÷ 10 = 30 MPG. Switch the same drive to thinking in metric and a car returning 30 MPG corresponds to roughly 7.8 L/100km. The tool does this cross-conversion with two constants under the hood: to turn L/100km into MPG it computes 235.214 ÷ value, and to turn km/L into MPG it multiplies by 2.35215. These factors fold together the litres-per-gallon and kilometres-per-mile ratios, so a single 30 MPG figure becomes the common yardstick no matter which unit you started in.

How the comparison bar reads your result

After computing efficiency, the tool converts it to an MPG equivalent and plots it against a benchmark of 30 MPG, the rough US combined average for new cars. The bar is scaled so that 60 MPG fills it completely, with a marker showing where the 30 MPG average falls. If your MPG equivalent meets or beats 30 the bar turns green and reports the percentage you are above average; below it, the bar turns amber and reports how far below you sit. In the 300-mile, 10-gallon example, 30 MPG lands exactly on the benchmark — neither above nor below. A truck returning 18 MPG would show amber at roughly 40% below average, while a hybrid at 50 MPG would show green at about 67% above.

Projecting your real fuel costs

The cost section turns efficiency into money. Enter your typical monthly distance and the local fuel price, and the calculator works out how much fuel that distance needs, then multiplies by price. The fuel-needed maths depends on the unit:

• MPG: gallons needed = monthly distance ÷ MPG.
• km/L: litres needed = monthly distance ÷ km/L.
• L/100km: litres needed = (monthly distance ÷ 100) × L/100km.

It then shows the monthly cost and multiplies by 12 for an annual figure. For example, a 30 MPG car driven 1,000 miles a month at $3.50 per gallon needs 1000 ÷ 30 ≈ 33.3 gallons, costing about $116.67 per month and $1,400 per year. Drop to a 20 MPG vehicle and the same driving costs about $175 a month — a $700 yearly difference that makes the case for efficiency concrete rather than abstract.

How to measure your mileage accurately

1. Fill the tank completely and note the odometer or reset the trip meter to zero.
2. Drive normally until you need fuel again — at least a few hundred miles for a stable reading.
3. Fill completely again and record the litres or gallons the pump dispensed; that is exactly the fuel you used over the distance shown on the trip meter.
4. Enter the distance and fuel into the calculator. Repeat over several tanks and average the results to smooth out variation.

A single tank can be misleading because driving conditions vary; an average over three or four fill-ups is far more representative of your real-world economy.

Common mistakes

• Mixing up US and UK gallons. A UK (imperial) gallon is about 20% larger than a US gallon, which is why UK MPG figures look so much higher for the same car. This tool's MPG mode uses US gallons; a "47 mpg" UK rating is not directly comparable.
• Not filling completely both times. The whole method relies on the tank being equally full at start and finish. A partial top-up throws the fuel-used figure off and skews the result.
• Comparing city and highway numbers. Economy swings widely with driving style and traffic. Compare like with like — don't pit a stop-start commute against a motorway cruise.
• Confusing the direction of L/100km. Remember that for L/100km, lower is better. A car at 5 L/100km is more efficient than one at 8, even though the larger number might instinctively feel like "more."