How Fuel Cost Is Calculated
Three numbers decide what a journey costs: how far you go, how much fuel the vehicle uses per unit of distance, and what that fuel costs.
Fuel used = Distance ÷ Fuel economy
Total cost = Fuel used × Price per unit
The complication is units. The United States measures economy in miles per US gallon, the United Kingdom in miles per imperial gallon — a gallon 20% larger — and most of the world in litres per 100 kilometres, which runs in the opposite direction: lower is better. The calculator converts everything internally to litres per 100 km, so any combination of inputs works.
Converting Between Economy Units
| From | To L/100 km | Example |
|---|---|---|
| US mpg | 235.215 ÷ mpg | 30 mpg = 7.84 L/100 km |
| Imperial mpg | 282.481 ÷ mpg | 30 mpg (UK) = 9.42 L/100 km |
| km per litre | 100 ÷ km/L | 12 km/L = 8.33 L/100 km |
Note that a car rated 30 mpg in Britain is less efficient than one rated 30 mpg in America, because the imperial gallon is larger. To convert US mpg to imperial mpg, multiply by 1.201.
Why MPG Is a Misleading Scale
This is the single most useful thing on this page. Miles per gallon is a reciprocal measure, so equal steps in mpg do not represent equal fuel savings. Over 10,000 miles at $3.50 a gallon:
| Upgrade | Gallons saved | Money saved |
|---|---|---|
| 10 → 15 mpg | 333 | $1,167 |
| 15 → 20 mpg | 167 | $583 |
| 20 → 25 mpg | 100 | $350 |
| 30 → 35 mpg | 48 | $167 |
| 40 → 50 mpg | 50 | $175 |
| 50 → 60 mpg | 33 | $117 |
Replacing a 10 mpg truck with a 15 mpg one saves ten times more fuel than replacing a 50 mpg car with a 60 mpg one, even though the second improvement looks twice as large. This is why L/100 km, which is linear in fuel consumed, is the more honest unit — and why fleet decisions should always start with the thirstiest vehicles.
What Actually Changes Your Real-World Economy
| Factor | Typical effect |
|---|---|
| Speed above 50 mph | −7% to −14% per additional 10 mph |
| Aggressive acceleration and braking | −10% to −40% |
| Under-inflated tyres | −0.2% per psi below spec |
| Roof rack or box | −10% to −25% at motorway speed |
| Air conditioning | −5% to −25%, worst in city driving |
| Extra 45 kg (100 lb) of load | −1% to −2% |
| Cold weather, short trips | −12% to −22% until the engine warms |
| Idling | 0 mpg — restart uses less fuel than 10 seconds of idling |
Aerodynamic drag rises with the square of speed and the power needed rises with the cube, which is why motorway speed dominates fuel use on long trips while stop-start behaviour dominates in town.
Fuel Is Not the Whole Cost
Running a car costs considerably more per mile than the fuel alone. Typical U.S. figures for an average sedan:
| Cost | Per mile |
|---|---|
| Fuel | $0.12 |
| Depreciation | $0.25 |
| Insurance | $0.08 |
| Maintenance and tyres | $0.09 |
| Registration, taxes and fees | $0.05 |
| Total | ~$0.59 |
This is why reimbursement rates — the U.S. federal mileage rate is around 67 cents — are several times the pure fuel cost. When comparing driving against another option, the fuel figure alone understates the true cost by roughly five times.
Frequently Asked Questions
Which fuel economy figure should I enter?
Your own, measured over several tanks: divide miles driven by gallons added. Manufacturer figures are laboratory results and are typically 10–20% optimistic in real driving.
How do I measure my real economy?
Fill the tank completely, reset the trip meter, drive normally, then fill completely again. Divide the trip distance by the fuel added. Repeat over three tanks and average.
Does premium fuel improve economy?
Only in engines designed to require it. In a car specified for regular fuel, premium delivers no measurable benefit and simply costs more.
How does this compare with an electric car?
Electricity is normally 3–5 times cheaper per mile. A typical EV uses about 0.3 kWh per mile, which at $0.15/kWh is $0.045 a mile against roughly $0.12 for a 30 mpg petrol car.