What will this trip actually cost in fuel?

How do I convert between MPG, L/100km, and km/L?

They measure efficiency in opposite directions — MPG and km/L are "distance per unit of fuel" (higher is better), while L/100km is "fuel per unit of distance" (lower is better) — which is why you can't just multiply by a fixed conversion factor between L/100km and the other two. This calculator handles the math internally based on whichever unit you select, so you don't need to convert manually.

Does this account for highway vs. city driving efficiency?

No — enter whichever efficiency figure best represents your actual trip; many vehicles list separate city and highway MPG or L/100km ratings. For a mixed trip, a combined or average rating will be more accurate than either extreme.

Why would I use this instead of just checking my car's trip computer?

A trip computer tells you what a trip already cost, or estimates range remaining — this tool is for planning ahead, before you drive: comparing routes, budgeting a road trip, or estimating commute costs at different fuel prices without needing to actually drive the distance first.

Worked example: a 500km round trip (1,000km total) at 6.5 L/100km and $1.50/liter needs 65 liters of fuel, for a total cost of $97.50.

What actually drives the number

Fuel cost is distance ÷ efficiency × price, so all three inputs matter equally — but efficiency is the one people estimate worst. Manufacturer figures are measured under laboratory conditions and real-world consumption is routinely 10-20% worse.

The reliable method is to measure it yourself: fill the tank, reset the trip meter, drive normally, then fill again and divide the distance by the fuel added. Two or three tanks gives a figure worth trusting far more than the brochure.

Speed costs more than most people think

Aerodynamic drag rises with the square of speed, and the power needed to overcome it with the cube. Most cars are near their most efficient around 50-60 mph, and consumption climbs sharply above roughly 70. Sustained motorway speed is usually the largest single factor in a long trip's fuel bill.

• A roof box or bike rack can cost 10-25% at motorway speed, even empty. Remove it when not in use.
• Under-inflated tyres raise rolling resistance measurably. Check monthly.
• Hard acceleration and braking waste energy as heat — smoother driving is the cheapest improvement available.
• Air conditioning costs a few percent; at motorway speed it is still cheaper than open windows, which spoil the aerodynamics.
• Short trips from cold are disproportionately expensive — an engine below operating temperature can use double its normal fuel.

Fuel is not the cost of driving

Depreciation, insurance, tax, servicing and tyres frequently exceed fuel over a year. For deciding whether to drive or take the train, fuel is the right marginal number. For deciding whether to own the car at all, it is a fraction of the picture — the Lease Calculator and Car Loan Calculator cover the larger fixed costs.

Comparing across units and fuels

Take care with units: US and imperial gallons differ by about 20%, so a US MPG figure is not comparable to a UK one. Litres per 100km is an inverse scale — lower is better, and the relationship to MPG is not linear.

For EVs the equivalent is kWh per 100km against your electricity tariff, and charging at home off-peak versus a rapid charger can differ by a factor of three or more — a bigger swing than anything in petrol pricing.