Fuel Mileage Calculator

Single fill-up

The honest method: fill the tank completely, drive, then fill completely again. The fuel you add the second time equals what you burned.

Multi-tank log

Log every fill-up. Tick full tank only when you filled to the brim. Partial fills are recorded honestly: their fuel counts toward the next full-tank interval — the log never invents economy for a partial fill on its own.

Fuel cost per distance

A light companion to the mileage above: what each kilometre or mile of driving costs in fuel. (For a full trucking operating-cost breakdown, see our dedicated tool.)

Worked examples

Example 1 - single fill-up

You fill to full, drive 400 miles, then fill to full again with 12 US gallons. Your mileage: 33.3 US mpg (7.06 L/100km, 40.0 UK mpg, 14.17 km/L).

Example 2 - multi-tank log

Three fills: 45 L (full), 20 L (partial), 25 L (full) over 800 km. The partial fill's fuel counts toward the closing full-tank interval, so economy = 45 L / 800 km x 100 = 5.63 L/100km (17.78 km/L).

Example 3 - fuel cost

At 7.06 L/100km with fuel at 1.70 per litre, 100 km costs 12.00 per 100 km (0.12 per km, 0.19 per mile).

Accuracy tips

• Always full-to-full. A partial fill leaves an unknown remainder in the tank; only a brimmed tank is a known starting point.
• Use the same pump and park the same way. Pump shut-off points and even a slope change how much "full" holds.
• Fill to the first click, not the brim of the filler neck — safer and more repeatable.
• Average 3–5 tanks in the multi-tank log. One tank is typically accurate to about ±5%; an average tightens to ±1–2%.
• Note the conditions. City vs highway, winter vs summer, roof box on/off — compare like with like or the numbers will mislead you.

Methodology — formulas, constants, and boundaries

Unit definitions used (exact, by international agreement): 1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, 1 imperial (UK) gallon = 4.54609 L. Conversions follow directly: L/100km = 235.2146 ÷ US mpg = 282.4809 ÷ UK mpg; UK mpg = US mpg × 1.2009.

Honest partial-fill rule

A partial fill-up can never yield an economy figure on its own, because the fuel already in the tank is unknown. This tool only measures economy over full-tank to full-tank intervals: for each interval, distance = odometer difference between two full fills, and fuel = the sum of all fuel added after the first full fill up to and including the closing full fill. Partial fills between them contribute their fuel to that total. The first full-tank entry is the baseline — no economy is computed for it alone.

What this tool is and is not (scope boundaries)

• vs Fuel Economy Converter: that tool converts between economy units (US mpg ↔ UK mpg ↔ L/100km ↔ km/L) and estimates trip cost from a known economy figure. It never measures economy from fill-up data — that is this tool's job. Measure here, convert there.
• vs Tire Pressure Calculator: its MPG tab estimates how much economy you lose from under-inflated tires (U.S. DOE rule of thumb: ~0.2% worse per 1 psi). It does not measure your car's real economy — this tool does, from your log.
• vs Road Trip Cost Calculator: an all-in trip planner (fuel + tolls + hotels + food + drive-vs-fly). It takes your economy as an input; this tool computes that economy.
• vs Fuel Distance: that tool answers the inverse question — "given my economy and tank size, how far can I drive?" This tool never estimates range; it only measures economy from distance + fuel.
• vs trucking operating-cost tools: the fuel-cost tab here is a deliberately light per-distance helper (fuel only). Full operating-cost analysis belongs to the dedicated trucking tool.