A 300 mile journey in a car returning 45 imperial MPG uses 6.67 gallons, which at £1.45 a litre — £6.59 a gallon — costs about £44 in fuel. That is roughly half the real cost of making the trip, because tyres, servicing, depreciation and the mileage share of insurance and tax roughly double the per-mile figure for most cars.
Government mileage allowances exist precisely because of that gap. A rate set at 45p a mile is not a fuel price; it is an estimate of the whole marginal cost of running a car for an extra mile.
Which economy figure are you holding?
Three exist for the same car and they do not convert by anything memorable.
| Measure | The same car |
|---|---|
| US MPG | 30 |
| Imperial MPG | 36.03 |
| Litres per 100 km | 7.84 |
A US gallon is 3.785 litres and an imperial gallon 4.546, so the same car returns about 20 per cent more MPG on paper in Britain than in America without burning a drop more or less. Comparing a US figure with a UK figure directly is one of the most common errors in car buying.
Why is MPG a misleading measure?
Because it is inverted. Equal improvements in MPG are not equal savings in fuel, and the effect is severe at the thirsty end of the range.
Going from 15 to 20 MPG over 12,000 miles saves 200 gallons. Going from 40 to 60 saves 100. The second improvement looks larger and saves half as much. That is why most of the world labels consumption as litres per 100 km, which is linear in what you actually buy — and why replacing the oldest, thirstiest car in a household saves more than upgrading the newest.
How do you measure it honestly?
Brim to brim over a full tank. Fill to the click, zero the trip, drive normally, fill to the click again, and divide the distance by the fuel that went in.
One tank is a sample rather than a measurement — it reflects that week, that route and that weather. Three or four consecutive tanks averaged is the figure worth quoting, and it is usually a little worse than the trip computer, which tends to flatter.
What does a long motorway trip cost?
More per mile than the official combined figure suggests, because sustained high speed costs more than the mixed cycle the test uses. Drag rises with the square of speed, so the last 20 km/h is disproportionately expensive.
A 1,200 km drive at 6.5 litres per 100 km uses 78 litres; at 1.85 a litre that is 144, or 36 each between four people. Add a roof box and expect 10 to 20 per cent worse — a genuinely large penalty for something that spends most of the year empty.
Four people and their luggage add weight, which costs less than the roof box does. Aerodynamics dominates at motorway speed and mass dominates in town, which is why a fully loaded car on a motorway is barely worse than an empty one.
Tolls, ferries and city charges belong in the same estimate and are usually left out of it. On a European road trip they can rival the fuel: a route through France, Italy and Switzerland can add a hundred euros in tolls and a vignette before anyone has bought a coffee.
Is it fair to split fuel equally?
Splitting the fuel equally is the usual convention and it undercharges the driver, who is also absorbing the wear, the depreciation and the risk. Splitting the mileage-rate figure equally overcharges the passengers, who did not buy the car.
The workable middle is to split fuel plus tolls between everyone including the driver, and to treat the rest as the driver’s contribution. It is not precise and it is transparent, which is what a shared cost needs to be.
Questions people ask
Does driving slower save fuel? Between roughly 80 and 110 km/h, substantially. Below that, engine load and gearing start to work against you, so there is an optimum rather than a straight line.
How often will I need to stop? Divide the usable tank by the consumption. A 45 litre usable tank at 6.5 L/100 km gives about 690 km, so a 1,200 km drive needs two stops rather than one.
Should I include the return journey? For a cost estimate, yes — and it is the single most common omission. A 300 mile trip is usually 600 miles of driving.
Is it cheaper to drive or take the train? Compare the train fare against the full per-mile cost rather than the fuel alone, and remember that the car cost splits between occupants while most train fares do not.
Do electric cars change the arithmetic? The structure is identical with kWh in place of litres, and the two costs diverge sharply because home charging and public rapid charging can differ by a factor of three or four per unit. An EV trip cost depends far more on where you charge than an petrol one depends on where you fill up.
Measure over several tanks, convert once to whatever unit you are comparing in, and cost the whole journey rather than the fuel. The gas mileage calculator and MPG calculator handle the economy figure in all three units, the fuel cost calculator turns it into money per journey, and the road trip cost calculator adds the refuelling stops and the split.