Cost per kilometre — the only fuel number worth tracking

# The mistake

Most car buyers obsess over MPG. Most drivers track fuel prices. Almost nobody tracks the number that actually matters for trip budgeting: cost per kilometre.

The marketed MPG is a single number that hides three variables (your car, your driving, the fuel price) inside one ratio. Cost per kilometre collapses all three into a number you can multiply by trip distance. Once you have it, you don't need to think about MPG again.

# The math

cost per km = (consumption ÷ 100) × fuel price
# L/100km example
6.5 L/100km × €1.85/L = €0.12/km
# US MPG example
30 MPG ≈ 7.84 L/100km (convert: 235.21 ÷ MPG)
30 MPG at $3.80/gal ≈ $0.127/mile

That's the number. €0.12/km, $0.127/mile, whatever your currency is. Multiply by trip distance to get trip cost. Done.

# Why the catalogue number lies

Manufacturer-quoted MPG (or L/100km) comes from a standardised test — the WLTP cycle in Europe, the EPA cycle in the US. The car is run on a dyno at fixed speeds for a fixed time. No AC. No passengers. No luggage. No wind. Highway-grade tyres at optimal pressure. The test was designed to produce reproducible numbers across manufacturers, not realistic numbers for any individual driver.

Real-world MPG runs 10-20 % worse than WLTP for most cars, sometimes more. The older NEDC test (Europe, pre-2017) was notoriously generous — Real-world divergence of 30-40 % was common. WLTP closed most of that gap, but didn't eliminate it.

# Track your own number

Five tanks of fuel is enough for a stable personal MPG estimate. The process:

1. Fill up. Reset the trip meter.

2. Drive normally for a tankful.

3. Refill to the same click. Note the litres and the trip distance.

4. Litres ÷ trip distance × 100 = your real L/100km.

5. Repeat over five tanks. Average.

The average across five tanks usually lands within ~5 % of your long-run real-world consumption. The first tank alone can be off by 15 % because tank-filling click points aren't perfectly consistent.

Use the Fuel Cost Calculator to plug your number plus current fuel price and get cost per km. Update it whenever fuel prices move materially (>10 %) or you change driving patterns (new commute, different highway speeds, winter tyres).

# A worked trip

A 600 km road trip in a car that does 6.5 L/100km at €1.85/L petrol.

600 km × 6.5 L/100km × €1.85/L
= 600 × 0.065 × 1.85
= 600 × €0.12025/km
= €72.15

If you know your cost per km in advance, that becomes just 600 × €0.12 ≈ €72. No formula to remember, no consumption to look up. Cost per km is the only number you need.

# Speed costs more than people think

The catalogue assumes a fixed test speed. Real driving doesn't.

Aerodynamic drag scales with the square of velocity. So going from 100 to 130 km/h doesn't increase fuel consumption by 30 % (the speed delta) — it increases it by roughly (130/100)² − 1 = 69 % more drag, which translates to ~20-30 % more fuel consumed per kilometre. The exact number depends on the car's drag coefficient and frontal area; smaller, slipperier cars suffer less.

Other real-world consumers:

• Climate control — AC adds 5-10 %; full-blast cabin heater adds 1-3 %. Eco-mode AC trades 1-2 °C of comfort for ~3-5 % consumption.
• Stop-and-go city driving vs steady highway — roughly doubles consumption per km. Engines spend a lot of fuel idling and accelerating from stops.
• Roof boxes and bike racks — 5-15 % extra at highway speeds because of frontal area.
• Underinflated tyres — every 0.5 bar below recommended pressure costs 1-2 % in fuel.
• Cold weather — cars use 10-20 % more fuel in winter (cold engines run rich, denser air increases drag, winter fuel blends have lower energy density).

# EVs change the calculation

For electric vehicles, the same math applies but the "fuel" is electricity and the price varies wildly with where you charge.

• Home overnight tariff: £0.05-0.15/kWh
• Public slow charger: £0.30-0.50/kWh
• Public fast charger (50 kW+): £0.50-0.80/kWh
• Highway DC fast charger (150 kW+): £0.70-1.00+/kWh

A typical EV does 15-20 kWh/100km. At home-charged £0.10/kWh, that's £0.015-0.020/km — about a fifth of a petrol car's cost. Public-fast-charged at £0.75/kWh, the same car costs £0.11-0.15/km — comparable to a thirsty petrol car. The vehicle's efficiency matters less than where you plug it in.

# Bottom line

• Track your own real-world consumption over five tanks, not the catalogue number.
• Compute cost per km once and update when prices move > 10 %.
• That single number times trip distance is your trip cost — no other math needed.
• If you drive an EV, the per-kWh price at your typical charging location matters more than the car's efficiency rating.

# Related tools

• Fuel Cost Calculator
• Currency Converter (for international trip planning)