Fuel Cost Calculator

Three ways of saying the same thing

Fuel efficiency is quoted differently depending on where you are, and the measures run in opposite directions — which is why comparing them by eye goes wrong.

Measure	Used in	Better is
Litres per 100 km	Europe, much of Asia, Australia	Lower
Kilometres per litre	India, Pakistan, Japan	Higher
Miles per gallon (US)	United States	Higher
Miles per gallon (UK)	United Kingdom	Higher

L/100 km measures fuel for a fixed distance. Miles per gallon measures distance for a fixed fuel. One is the inverse of the other, so a car that looks twice as good on one scale is not twice as good on the other.

The calculator above shows all four for whatever you enter, so you can sanity check a figure against the one you are used to.

Converting between them

• US mpg → L/100 km: divide 235.215 by the mpg figure
• UK mpg → L/100 km: divide 282.481 by the mpg figure
• km/L → L/100 km: divide 100 by the km/L figure

So 30 US mpg is 235.215 ÷ 30 = 7.84 L/100 km.

The two constants differ because the gallons do. A UK gallon is 4.546 litres and a US gallon is 3.785 — about 20% bigger.

The mpg trap

Because the gallons differ, the same car gets a better mpg number in Britain than in America.

	US rating	UK rating	L/100 km
Small hatchback	33.3 mpg	40 mpg	7.06
Efficient saloon	40 mpg	48 mpg	5.88
Large SUV	20 mpg	24 mpg	11.76

Nothing about the car changed. Only the size of the gallon did. So when comparing cars across a used-import listing or a review from another country, convert first — a “45 mpg” British car is about 37 US mpg.

What a journey actually costs

The arithmetic is simple once the units agree:

Litres used = distance in km × (L/100 km) ÷ 100

A 500 km drive in a car using 8 L/100 km:

1. 500 × 8 ÷ 100 = 40 litres
2. At 280 per litre: 40 × 280 = 11,200

Split between four people that is 2,800 each — which is the number worth having before agreeing to drive everyone to a wedding.

Why your real consumption is worse than the sticker

Official figures come from standardised laboratory tests. Real driving does not resemble a laboratory:

• Speed. Air resistance rises with the square of speed. Going 120 km/h instead of 100 can cost 15–20% more fuel.
• Stop-start traffic. Accelerating from rest is the most expensive thing an engine does.
• Air conditioning. Up to 10% in hot conditions.
• Tyre pressure. Under-inflated tyres can add several percent, and they are the easiest thing on this list to fix.
• Load and roof racks. An empty roof box can cost 10% on the motorway.
• Short trips from cold. An engine below operating temperature uses markedly more fuel — the first few kilometres are the thirstiest of any journey.

Measuring your own, properly

Trip computers are estimates and usually optimistic. To get a real figure:

1. Fill the tank completely and note the odometer.
2. Drive normally until the tank is at least half empty.
3. Fill completely again, noting the litres that went in and the distance driven.
4. Litres ÷ kilometres × 100 = your true L/100 km.

Average two or three tanks. One tank can be skewed by a single long motorway run or a week of school runs, and the whole point is to get a number you can plan with.