Finance FIELD NOTES · 3 MIN READ
How to Measure Fuel Economy: The Fill-Up Method Behind MPG and L/100km
Why fuel economy can only be measured between two full fill-ups, and how the MPG, km/L and L/100km unit families relate.
Your car's advertised mpg was measured on a laboratory dyno. Your real fuel economy is a ratio only you can observe — and measuring it takes two full fill-ups and a trip odometer, not a dashboard estimate. One careful fill-up-to-fill-up measurement beats every guess.
The fill-up method
Fuel economy is the ratio of distance driven to fuel consumed, and the only moment you truly know either quantity is at the pump. The protocol: fill the tank completely and reset the trip odometer; drive normally; at the next fill-up record the miles on the trip odometer and the gallons that went in. Then:
mpg = miles driven / gallons addedExample: 320 miles on 11.2 gallons gives 320 / 11.2 = 28.6 mpg. Two details make or break the measurement: both fill-ups must be full (otherwise you do not know how much fuel the distance actually consumed), and the span should cover your normal mix of driving — a tank of all-highway will flatter a city car. Filling to the same reference point at both ends is what makes the subtraction valid: whatever you pump at the second stop is precisely what the engine burned since the first.
Two families of units, one truth
The world divides into distance-per-fuel and fuel-per-distance. MPG and km/L put distance on top — higher is better. Litres per 100 km puts fuel on top — lower is better. They are reciprocals, bridged by fixed conversions: 1 mile = 1.609344 km and 1 gallon = 3.78541 litres, which reduces to one handy constant for US units:
L/100km = 235.215 / mpgOur 28.6-mpg fill-up is therefore 235.215 / 28.57 = 8.2 L/100km (12.1 km/L). The conversion runs both ways: a metric fill-up of 520 km on 38 litres is 13.7 km/L, which is 7.3 L/100km and 32.2 mpg. The calculator normalizes every input to a common basis, so a single tank measured in any units lands in all four figures at once — it also reports miles per litre (7.5 here) for drivers who buy fuel by the litre but think in miles.
From economy to cost per mile
Economy becomes money the moment you add a fuel price. At $3.60 a gallon, the fill-up cost 11.2 × 3.60 = $40.32, which over 320 miles works out to 12.6 cents per mile — or 7.8 cents per kilometre. That division (cost ÷ distance) is the most useful number a tank produces, because it scales to any trip. The pricing side of this math — cost = distance / efficiency × price — is worked through in fuel cost calculation; what this measurement adds is the honest efficiency figure those calculators must assume.
Why your number will differ from the sticker
The window sticker comes from standardized lab cycles; your measurement absorbs everything the lab excludes: cold starts, idling, roof racks, under-inflated tires, aggressive acceleration, headwinds, winter fuel blends. A 10–15% gap is normal, and city-weighted driving can exceed it. Track three or more consecutive fill-ups and average them — total miles divided by total gallons — before concluding anything is wrong with the car. Single tanks are noisy; trends are not. A sudden drop across several tanks is a maintenance signal worth acting on.
Try it yourself
Enter your last fill-up in the Fuel Economy Calculator — distance and fuel in either unit system, plus an optional price — and read mpg, km/L, L/100km, trip cost and cost per mile at once. Then price a planned trip with the Fuel Cost Calculator, or a multi-leg route in the Fuel Cost with Stops Calculator.