Fuel Cost for Any Trip: Distance, MPG, and Pump Price Made Simple

Three numbers decide what a drive costs in fuel: how far you go, how much your car drinks, and what the pump charges. The trouble is they almost never arrive in matching units. Your odometer reads kilometres, the car review quotes MPG, and the receipt prints a per-litre price. Most people give up and guess — and road-trippers who guess almost always guess low.

This guide walks through the math, the unit traps that quietly break it, and how to get a defensible number for a road trip, a commute, or a carpool you need to settle.

The one calculation that matters

Fuel cost is just litres burned times price per litre. Everything else is unit conversion feeding those two values.

Litres burned come from distance and economy. If your car does 6.8 L/100km and you drive 480 km, you burn 480 ÷ 100 × 6.8 = 32.6 litres. Multiply by a €1.85/litre pump price and the trip costs about €60 one way, €120 round trip. That is the whole engine.

The reason it feels harder than that is that the three inputs rarely share a unit system. So before the multiplication can happen, distance has to become kilometres, economy has to become litres-per-distance, and price has to become per-litre. Do those three conversions consistently and the answer falls out. Do even one of them in the wrong unit and you can be off by a factor of four.

MPG and L/100km are reciprocals, not a scale

Here is the trap that catches almost everyone. MPG and L/100km do not move on a straight line — they are reciprocals. Higher MPG is better; higher L/100km is worse, and the gap between two figures means different things at each end of the scale.

Improving from 8 L/100km to 6 saves far more fuel than improving from 6 to 4, even though both look like "2 units". In MPG terms, going from 40 MPG to 20 MPG more than doubles the fuel you burn per 100 km — a bigger real jump than dropping from 20 to 10. If you compare cars in raw MPG steps, your intuition about savings will be wrong.

The conversion itself is: L/100km = (100 × litres-per-gallon) ÷ (MPG × 1.609). So 30 US MPG works out to about 7.84 L/100km, and 6 L/100km is about 39.2 US MPG. When money is the question, compare in L/100km or directly in cost — never in MPG格 steps. A quick way to sanity-check any unit conversion alongside fuel economy is the unit converter, which keeps the underlying factors visible.

US gallons and UK gallons differ by 20%

The second trap hides inside the word "MPG" itself. A US gallon is 3.785 litres. A UK (imperial) gallon is 4.546 litres — about 20% bigger. That means "40 MPG" describes two different cars depending on where the review was published.

On a British spec sheet, 40 MPG means the car covered 40 miles on 4.546 litres. On a US review, 40 MPG means 40 miles on only 3.785 litres. Same number, but the US car is actually the thriftier one, because it went the same distance on less fuel. The same engine looks roughly 20% more efficient on a British sheet than an American one, purely from the gallon definition.

Practical rule: UK and Australian sources mean imperial gallons; US sources mean US gallons. When a spec sheet just says "MPG", check the publication before you compare two cars. A converter that keeps US MPG and UK MPG as separate units — like the one built into the fuel cost calculator — stops you from silently comparing an American figure against a British one.

A worked example

Say you are pricing a 300-mile drive in a car rated at 28 US MPG, with gas at $3.50 per US gallon. Gallons burned: 300 ÷ 28 = 10.7 gallons. Cost: 10.7 × $3.50 ≈ $37.50 one way. Round trip doubles it to about $75.

If four of you are in the car, the round-trip $75 splits to roughly $18.75 each. That is the number a passenger actually owes the driver — not "just give me twenty bucks", but a figure everyone can see and agree on. Splitting and percentage math like this is exactly what a percentage calculator is good for when you want to double-check the share by hand.

Commuting, reimbursement, and the per-mile rate

Trip cost is the headline, but the per-kilometre and per-mile figures are where fuel math earns its keep day to day.

For a commute, the per-mile rate times your annual distance turns a daily nuisance into an annual line item. A car at 7.5 L/100km versus one at 6.0 L/100km might look like a rounding difference until you run your real yearly mileage through both at today's price — then it becomes "this one costs me €310 more a year", which is the number that actually moves a buying decision.

For mileage reimbursement, the per-mile cost — derived from your logged distance, your car's real economy, and the receipt price — gives you a defensible expense line instead of a guess. Because fuel cost scales linearly with distance, the per-mile rate is the same whether you costed a one-way or round-trip distance, so you can multiply your own logged miles by it directly.

What I actually do before a long drive

I stopped trusting my gut on this after a coast trip a couple of years back where I was sure the train was the splurge option. It wasn't. When I finally ran the numbers — distance, my car's honest highway economy, the motorway pump price — the drive was within a few euros of two train tickets, and that was before parking. Now I price every trip over a couple hundred kilometres the same boring way: distance, economy, price, round-trip on. It takes ten seconds and it has talked me out of more "obviously cheaper" drives than I expected.

Common mistakes to avoid

Comparing US MPG against UK MPG as if they were equal. They are not; the UK gallon is 20% bigger. Confirm which gallon the source used first.
Treating L/100km and MPG as linear. They are reciprocals — compare in L/100km or cost when money is the point.
Mismatching the price unit. Set "price per US gallon" but type the per-litre price and your cost comes out roughly 3.8× too low. Check the unit dropdown matches the receipt.

Get the three inputs and their units right and the answer is honest. Get one unit wrong and you are off by a multiple — which is exactly why a tool that normalises everything to litres and kilometres internally beats doing it in your head.

Made by Toolora · Updated 2026-06-13