Fuel cost for a trip: L/100 km, mpg and splitting it fairly

To work out the fuel cost of a trip, convert the car's efficiency to litres per 100 kilometres, multiply by the distance divided by 100 to get the litres needed, then multiply by the price per litre. Efficiency is quoted in several units, and the most common mistake is mixing US and UK miles per gallon. Convert first, then calculate, and the rest is simple arithmetic.

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Efficiency units

Efficiency appears in four common forms, and each can be converted to litres per 100 km (L/100 km):

• L/100 km. Litres used for every 100 km. A lower number is better. Common across Europe and in many other countries.
• km/L. Kilometres for every litre. A higher number is better. The reciprocal of L/100 km, scaled by 100: km/L = 100 ÷ (L/100 km).
• US mpg. Miles per US gallon. A mile is 1.609 km and a US gallon is 3.785 litres. L/100 km = 235.2 ÷ US mpg.
• UK mpg. Miles per imperial (UK) gallon. A UK gallon is 4.546 litres. L/100 km = 282.5 ÷ UK mpg.

The two mpg figures look similar but are not. A UK gallon is larger than a US gallon, so the same car gets a higher UK mpg figure than US figure. Check which one the car's specification or the label uses before you convert.

Worked example

Suppose a car is rated at 30 mpg (US). Convert it:

• L/100 km = 235.2 ÷ 30 = 7.84 litres per 100 km.
• km/L = 100 ÷ 7.84 = 12.76 km per litre.

Check the conversion by working in miles and gallons. 30 miles is 30 × 1.609 = 48.27 km. That uses one US gallon, or 3.785 litres. So 3.785 litres for 48.27 km is 3.785 ÷ 48.27 × 100 = 7.84 litres per 100 km, which matches.

Now the trip. The one-way distance is 400 km, and fuel costs $1.65 per litre:

• Litres needed: 400 ÷ 100 × 7.84 = 31.36 litres.
• Cost one way: 31.36 × 1.65 = $51.74.
• Round trip, 800 km: 62.72 litres, which costs $103.49.

Splitting the fuel cost

Fuel is a shared cost when several people travel together. A simple split works for most trips:

1. Decide who is sharing. If four people travel and all four share the cost, divide by four.
2. Decide whether the driver shares the cost. Many groups do, because the car is used for the trip, but some drivers ask for a share of the fuel only.
3. Divide the total, rounding each share to the cent.

For the round trip, $103.49 split four ways is $25.87 each, which adds back to $103.48, so one person pays the odd cent. A split bill calculation handles that rounding for you, and the same method works for tolls and parking.

Tolls, parking and wear

Fuel is only part of the cost of a journey. Tolls, parking and wear on the car are additional costs. Decide at the start whether they go into the split, since they are often larger than people expect on a long route. Keep receipts, and share them with the group if you are paying for things on behalf of others.

Efficiency changes with the trip

The figure on a label is measured under standard conditions, and real driving differs. Speed, load, air conditioning, tyre pressure, and whether the trip is mostly motorway or city traffic all change the result. For planning, add a margin, or use the efficiency you have recorded on similar trips.

A worked comparison shows the scale of this. A car rated at 40 UK mpg uses 282.5 ÷ 40 = 7.06 litres per 100 km. On an 800 km round trip that is 56.48 litres. If the same car uses 10% more in real conditions, the trip needs about 62 litres, so a plan based on the label can be about 5.6 litres short.

Steps for a trip budget

1. Find the efficiency on the car's documents, and note its unit.
2. Convert to L/100 km, using the factors above.
3. Multiply by the distance divided by 100, for the litres needed.
4. Multiply by the price per litre, using a price you have checked for the route.
5. Add tolls and parking, then split the total among the people sharing it.

Edge cases

• Combined figures. A label may show a combined figure for city and motorway driving. Use it as an estimate, and adjust for your own route.
• Electric and hybrid cars. The conversion works for petrol and diesel only. Electric cars use kilowatt-hours per 100 km, and hybrids need the petrol figure for the petrol part of the trip.
• Price changes. Fuel prices move, so use the price when you are filling up, and say which price you used.
• Rounding the litres. Round the litres you need up, not down, so that a small shortfall does not catch you out on the road.

Quick answers

How do I convert US mpg to L/100 km?

Divide 235.2 by the mpg figure. A car rated at 30 US mpg uses 7.84 litres per 100 km.

Why are UK and US mpg figures different for the same car?

A UK gallon is larger than a US gallon, so the same fuel covers more miles per UK gallon. The two figures are both correct in their own units.

How do I split fuel between passengers fairly?

Divide the total by the number of people sharing it, round to the cent, and give the odd cent to one person. Decide in advance whether the driver shares the cost.