Fuel Cost Calculator — Road Trip Gas Cost

How the Fuel Cost Calculator Works

1. 1Select your unit system — US (miles and gallons), UK (miles and litres), or metric (km and L/100km).
2. 2Enter the trip distance in miles or kilometres.
3. 3Enter your vehicle's fuel economy — MPG from the dashboard or owner's manual, or L/100km for metric vehicles.
4. 4Enter the current fuel price per gallon or litre, and the number of trips if calculating a round trip or commute total.

MPG vs L/100km — Which Fuel Economy Measure Is More Useful?

MPG (miles per gallon) is intuitive — bigger is better. But it is non-linear, which means the fuel saving from going from 15 to 20 MPG is much larger than going from 40 to 50 MPG, even though both are 5 MPG improvements. L/100km (litres per 100 km) is linear and proportional — halving L/100km always halves fuel cost. For comparing vehicles with very different efficiency levels, L/100km gives a clearer picture of real-world saving.

What Actually Affects Your Fuel Economy

Fuel economy is not a fixed property of your car — it is the outcome of a constant battle against a handful of physical forces, and understanding them explains why the figure on the window sticker rarely matches reality. The biggest villain at speed is aerodynamic drag, which rises with the square of your velocity: doubling your speed roughly quadruples the drag force, which is why fuel economy falls off a cliff above about 60–70 mph. This single fact means a relaxed motorway pace can use dramatically less fuel than pushing hard, with only a small time penalty.

The other forces matter too. Rolling resistance — the friction of the tyres on the road — is why under-inflated tyres cost you fuel, and weight increases both the energy needed to accelerate and the rolling resistance, so a boot full of clutter or a roof box quietly raises your consumption. On top of the physics sits engine efficiency: engines have a sweet spot of load and revs where they convert fuel to motion most effectively, which is why steady cruising beats stop-start city driving and why aggressive acceleration — forcing the engine out of its efficient zone — is so wasteful. Smooth, anticipatory driving at a moderate speed, with correct tyre pressure and no unnecessary weight, is the practical sum of all this.

The True Total Cost of Driving

Fuel is the most visible cost of driving because you pay it at the pump, but it is far from the only one — and often not even the largest. The biggest cost of car ownership for many people is actually depreciation: the value a car loses over time, which can dwarf a year's fuel spending yet goes unnoticed because you never write a cheque for it. Then come insurance, maintenance and repairs, tyres, road tax, and parking. Motoring organisations that total these up typically find fuel is only a fraction of the real per-mile cost of running a car.

This matters for real decisions. When you use this calculator to price a trip, remember it is showing the marginal fuel cost — the extra you spend by making this journey — which is exactly the right number for comparing driving against a train ticket, or deciding whether a detour is worth it. But when comparing whole vehicles or lifestyles (car versus no car, a commute versus working from home), the fuller picture including depreciation and fixed costs is what counts. Knowing which cost you are actually weighing keeps the comparison honest.

Electric vs Petrol Running Costs

The rise of electric vehicles has made “fuel” cost more interesting to compare. An EV is usually much cheaper to “refuel” per mile, because electricity delivers energy to the wheels far more efficiently than a combustion engine converts petrol — and charging at home, especially on a cheap overnight tariff, can cost a fraction of the equivalent petrol. EVs also tend to have lower maintenance costs, with fewer moving parts, no oil changes, and less brake wear thanks to regenerative braking.

The comparison is not entirely one-sided, though. EVs typically have a higher purchase price (and so more depreciation to absorb), public rapid-charging can cost far more than home charging — sometimes approaching petrol-per-mile costs — and running costs depend heavily on your electricity tariff and how you charge. To compare fairly, work out the cost per mile for each: for a petrol car, divide the fuel price by the miles per gallon (or use this calculator); for an EV, divide your electricity price by the miles it travels per unit of energy. Doing that on your own real numbers, rather than trusting a headline, is the only way to know which is genuinely cheaper for your driving.