Fuel Economy Converter

Fuel economy is expressed two ways: distance per volume (miles per gallon, kilometers per liter) and volume per distance (liters per 100 km). The first scales linearly with efficiency; the second is inversely proportional.

Given a value in any of those units, the converter returns the others. It distinguishes US gallons (3.785 L) from imperial gallons (4.546 L), which differ by about 20% — a difference that quietly inflates UK car ads when read by American audiences.

Conversion factors are fixed by definition. 1 US gallon = 3.78541 L exactly (the gallon is defined in terms of the cubic inch and the inch in terms of the meter). 1 imperial gallon = 4.54609 L. 1 mile = 1.609344 km. To convert MPG (US) to L/100km: divide 235.215 by the MPG value. The 235.215 constant comes from (3.78541 × 100) / 1.609344. So 30 MPG (US) is 235.215/30 = 7.84 L/100km. The same operation in reverse: L/100km to MPG is 235.215 divided by L/100km.

A worked example. A car rated at 6.5 L/100km (a typical European compact). MPG (US) = 235.215 / 6.5 = 36.2 MPG. MPG (imperial) = 282.481 / 6.5 = 43.5 MPG. Same physical efficiency, three different headline numbers depending on jurisdiction. The 7-MPG gap between US and imperial figures is purely the gallon difference — a UK ad claiming 50 MPG describes a car that an American driver would experience as 41.6 MPG.

Limitations and edge cases. EPA combined ratings are weighted averages of city and highway test cycles run on dynamometers under controlled conditions. Real-world economy is usually 10 to 20 percent lower for gas vehicles, more in cold weather and stop-go traffic. EVs use MPGe, converting electrical energy to gasoline equivalent at 33.7 kWh per gallon — a useful comparison metric but not an operating-cost predictor since electricity and gas prices move independently. Hybrids report city MPG higher than highway because regenerative braking favors stop-and-go. Plug-in hybrids report a blended efficiency that depends entirely on how often the driver plugs in; the published number assumes a typical mix that may not match yours.