Why do US calculators give a different answer for my mpg?

A UK gallon is 4.54609 litres and a US gallon 3.785411784, so the same car shows about 20% more mpg in UK figures. Type 45 UK mpg into a calculator built on the US gallon and it treats the car as more economical than it is, so the trip comes out about a sixth too cheap. This page uses the imperial gallon for mpg UK and names it in the working.

How do I work out fuel cost per mile in the UK?

Multiply the price per litre by 4.54609 and divide by the mpg. At 1.45 a litre and 45 mpg that is 1.45 × 4.54609 ÷ 45 = 0.1465, about 14.6p a mile. The Per mile and per km row shows it for your own numbers.

Does it work for diesel and for a van?

Yes. Enter the diesel price per litre and the van's own mpg. The sum depends only on economy and price, not on the fuel or the vehicle.

UK Fuel Cost Calculator

Britain measures journeys in miles, cars in imperial mpg and fuel by the litre, and this page opens set up that way, with each conversion shown in the working. It starts on 200 miles at 45 mpg and 1.45 a litre: 20.205 litres and 29.30. Enter the price in pounds, so 145.9p a litre is 1.459.

One way fuel cost29.3020.205 L over 200 miles

Distance

Efficiency

Measured in

Fuel price

Quoted

Sharing

Trip

Round trip

Compare

Second vehicle

Enter the price in whatever currency you buy fuel in. The answer comes back in the same one, because the arithmetic never touches an exchange rate.

Fuel neededone way
20.205 L (4.444 UK gal)
Total costfuel only, no tolls or parking
29.30
Per personone person
29.30
Per mile and per kmthe same money over two rulers
0.1465 per mile, 0.0910 per km
The workingevery conversion, in full
200 mi ÷ 45 mpg (UK) = 4.4444444 UK gal; 4.4444444 UK gal × 4.54609 = 20.204844 L; 20.204844 L × 1.45 = 29.30

Two gallons, and one unit that runs backwards. Choose the gallon used by your fuel price and efficiency figure. L/100km measures fuel per distance, so a lower number means less fuel used, and it multiplies where mpg divides. Every conversion the answer needs is written into the working above with the constant it used, so you can check which gallon went where.

Common questions

Why do US calculators give a different answer for my mpg?: A UK gallon is 4.54609 litres and a US gallon 3.785411784, so the same car shows about 20% more mpg in UK figures. Type 45 UK mpg into a calculator built on the US gallon and it treats the car as more economical than it is, so the trip comes out about a sixth too cheap. This page uses the imperial gallon for mpg UK and names it in the working.
How do I work out fuel cost per mile in the UK?: Multiply the price per litre by 4.54609 and divide by the mpg. At 1.45 a litre and 45 mpg that is 1.45 × 4.54609 ÷ 45 = 0.1465, about 14.6p a mile. The Per mile and per km row shows it for your own numbers.
Does it work for diesel and for a van?: Yes. Enter the diesel price per litre and the van's own mpg. The sum depends only on economy and price, not on the fuel or the vehicle.

Exact arithmetic on the numbers entered: distance divided by efficiency, times price. Real consumption varies with speed, load and weather, so treat the result as the figure your own inputs imply.