Measuring economy with the full-tank method
A car’s trip computer gives an estimate; the pump and the odometer give a measurement. The standard way to take it is the full-tank (tank-to-tank) method, which the US government’s fueleconomy.gov uses for its own mileage log: it needs two consecutive fill-ups where the tank is filled completely.
- Fill the tank until the pump nozzle clicks off the first time. Do not top up after the click; stopping at the same point each time is what makes the method consistent.
- Reset the trip meter to zero, or write down the odometer reading.
- Drive normally until you next need fuel.
- Fill up again to the first click. Note the fuel added (liters or gallons, from the pump display or receipt) and the distance on the trip meter.
- Enter that distance in Distance since last fill-up and the fuel in Fuel added to fill the tank, each with its unit.
The logic is simple: the second fill-up puts back exactly the fuel the engine burned since the first one, so distance divided by that fuel is the economy over that stretch. If you skip a fill-up, or fill only part of the tank in between, the calculation breaks, because the fuel you enter no longer matches the distance.
The tool shows the answer in L/100 km (lower is better), and also as km per liter, mpg (US) and mpg (UK) (higher is better for all three).
Formulas for each unit
The calculator converts your distance to kilometers and your fuel to liters first, using a mile of 1.609344 km, a US gallon of 3.785411784 L and an imperial gallon of 4.54609 L, so all four results describe the same measurement.
Worked examples
- L/100 km: 38.5 ÷ 520 × 100 = 7.404.
- km/L: 520 ÷ 38.5 = 13.51.
- Converted to miles and gallons, the same result is 31.77 mpg with US gallons and 38.15 mpg with imperial gallons.
- mpg (US): 312 ÷ 10.4 = 30.
- In metric terms that is 7.84 L/100 km.
- A UK driver would call the same car a 36 mpg car, because the imperial gallon is bigger.
- The log shows 610 km on 41.2 L, 455 km on 36.8 L and 580 km on 39.9 L. One at a time they give 6.754, 8.088 and 6.879 L/100 km.
- Add the distances: 610 + 455 + 580 = 1,645 km. Add the fuel: 41.2 + 36.8 + 39.9 = 117.9 L.
- Enter the totals in the calculator: 117.9 ÷ 1,645 × 100 = 7.167 L/100 km.
- The plain average of the three readings would be 7.24, which gives the shortest tank too much weight.
Why one tank is not enough
A single measurement can be off for reasons that have nothing to do with the car. Pumps click off at slightly different levels, the car may stand on a slope, and a tank that was mostly highway driving will look much better than one spent in winter city traffic. Natural Resources Canada notes that real consumption depends on how, where and when you drive, and one tank captures only one slice of that.
Logging several fill-ups smooths this out. The right way to combine them is the way the third example does: total distance divided by total fuel. Averaging the individual results gives a different, misleading number, especially with mpg. Suppose one tank covers 300 miles on 10 US gallons (30 mpg) and the next covers 300 miles on 20 gallons (15 mpg). The mean of 30 and 15 is 22.5 mpg, but the car actually drove 600 miles on 30 gallons, which is 20 mpg.
Errors that distort a mileage reading
- Topping off past the click. Squeezing in extra fuel at one fill-up and not the next adds liters to one reading and removes them from another.
- Using the trip computer’s figure as the distance or fuel. Use the odometer or trip meter for distance and the pump’s figure for fuel; the dashboard average is itself an estimate.
- Entering the tank’s capacity instead of the fuel added. The tank holds, say, 50 L, but if you put in 38.5 L, the car used 38.5 L.
- Mixing US and imperial gallons. Older UK logbooks and UK mpg figures use imperial gallons, while US pumps sell US gallons. Pick the right one in Fuel unit.
- Ignoring a missed fill-up. If you forgot to enter one stop but still have its receipt, merge the two periods: add both distances and both fuel amounts. Without the receipt, start a fresh measurement at the next full tank.
Electric and plug-in hybrid cars need a different measure, energy per distance (kWh/100 km or miles per kWh), because charging is not measured in liters or gallons.
Sources
- My MPG Help, U.S. Department of Energy and U.S. Environmental Protection Agency (fueleconomy.gov).
- Understanding fuel consumption ratings, Natural Resources Canada.
- Weights and Measures Act 1985, Schedule 1 (definition of the gallon), legislation.gov.uk (The National Archives).