MPG Calculator

What is this?

An MPG Calculator works out a vehicle's fuel economy from the distance driven and the fuel used, and converts between miles per UK gallon, miles per US gallon, litres per 100 km and kilometres per litre. It also turns that economy into a cost per mile and the fuel cost of a specific journey.

How to Use the MPG Calculator

An MPG calculator works out your vehicle's fuel economy from the distance you drove and the fuel you used, and converts it between every unit in common use - miles per UK gallon, miles per US gallon, litres per 100 km and kilometres per litre.

The unit that catches people out is the gallon. An imperial gallon is 4.54609 litres and a US gallon is 3.785411784, so the same car reads about 20% higher in UK mpg than US mpg. A car quoted at 40 mpg in the UK is 33.3 mpg in the US - identical car, different gallon.

1. Measure the Distance

Fill the tank, reset the trip meter, then drive normally. The distance on the trip meter when you next fill up is what you need. If your car has no trip meter, note the odometer at each fill and subtract.

2. Record the Fuel Used

The amount that goes in at the second fill is the fuel used over that distance, provided both fills take the tank to the same point. Enter it in litres or gallons.

3. Add the Fuel Price

Optional, but it turns the economy figure into a cost per mile - the number that actually matters for budgeting. Enter the price per litre or per gallon.

4. Read Every Unit at Once

The result gives UK mpg, US mpg, L/100km and km per litre together, so a figure quoted in any of them can be compared directly.

5. Cost a Journey

Switch to the Journey cost tab, enter the distance, your vehicle's known economy and the fuel price, and it works out the fuel needed and what the trip will cost.

Key Formulas Used in the Calculator

Miles Per Gallon

$\text{MPG} = \frac{\text{Miles}} {\frac{\text{Litres}}{\text{Litres per Gallon}}}$

An imperial gallon is 4.54609 litres and a US gallon is 3.785411784. Driving 300 miles on 35 litres is 38.97 UK mpg or 32.45 US mpg.

Litres Per 100 km

$\text{L/100 km} = \frac{\text{Litres}}{\text{Kilometres}} \times 100$

This runs the opposite way to mpg - a lower number is better. The same 300 miles on 35 litres is 7.25 L/100km.

Converting Between Them

$\text{MPG}_{\text{UK}} \times \text{L/100 km} = 282.48$

$\text{MPG}_{\text{US}} \times \text{L/100 km} = 235.21$

Because one measures distance per fuel and the other fuel per distance, they do not convert linearly - their product is a constant. Divide the constant by one figure to get the other.

UK to US MPG

$\text{MPG}_{\text{US}} = \text{MPG}_{\text{UK}} \times \frac{3.785411784}{4.54609}$

That factor is 0.8327, so 40 UK mpg is 33.31 US mpg. Going the other way, multiply US mpg by 1.20095.

Benefits

• Handles UK and US gallons correctly rather than assuming one

• Shows UK mpg, US mpg, L/100km and km per litre together

• Accepts distance in miles or kilometres and fuel in litres or either gallon

• Turns fuel economy into a cost per mile and per 100 miles

• Costs a specific journey from a known mpg figure

• Explains why the UK and US figures differ by 20%

• Shows the working, including the constants used

When & Where to Use

• Checking whether your car matches its advertised economy

• Comparing a US review figure against a UK one

• Working out the fuel cost of a long journey before setting off

• Tracking economy across several tanks to spot a developing fault

• Comparing two cars quoted in different units

• Claiming mileage expenses at an accurate cost per mile

• Seeing what a change in driving style is worth per year

Who Should Use This Calculator

The MPG Calculator is for drivers tracking running costs, anyone comparing vehicles quoted in different units, fleet and company car managers, and people claiming mileage expenses.

Tips to Get the Best Deal

Always check which gallon a quoted mpg figure uses before comparing

Fill to the same point both times, or the fuel figure will be wrong

Average several tanks - a single tank is heavily affected by the roads you drove

Lower is better for L/100km, the opposite of mpg

Cost per mile is the figure to compare when budgeting, not mpg

Motorway driving usually gives better economy than town driving

A sudden drop in economy can be an early sign of a mechanical problem

Pro Tips

• Always check which gallon a quoted mpg figure uses - UK is about 20% higher than US.

• Fill to the same point both times or the fuel figure will be wrong.

• Average several tanks; a single tank reflects that journey's conditions.

• Lower is better for L/100km, the opposite of mpg.

• Cost per mile is the figure to compare when budgeting, not mpg.

• Trip computer readings are usually a few percent optimistic.