Calculate MPG or L/100km from distance travelled and fuel used.
MPG (miles per gallon) and L/100km are inversely related: L/100km = 235.214 ÷ MPG. The constant 235.214 comes from converting miles to kilometres and gallons to litres. Because of the inverse relationship, improving from 10 to 20 MPG saves twice as much fuel per mile as improving from 20 to 40 MPG.
Miles per gallon
MPG = distance in miles ÷ fuel in US gallons
L/100km
L/100km = (litres ÷ km) × 100
Conversion between MPG and L/100km
L/100km = 235.214 ÷ MPG
MPG measures how far you go per unit of fuel (higher is better), while L/100km measures how much fuel you use per unit of distance (lower is better). Mathematically, one is the reciprocal of the other, scaled by unit conversions. This is why gas mileage improvements are subject to diminishing returns measured in MPG.
Fill your tank completely and note the odometer. Drive normally. When you next fill up, note the number of gallons (or litres) it takes to re-fill and the new odometer reading. Divide the miles driven by gallons used, or multiply (litres / km) by 100.
For passenger cars in the US, EPA considers 30+ MPG (≤7.8 L/100km) good for a non-hybrid. Hybrids typically achieve 50–70 MPG (4.7–3.4 L/100km). SUVs and trucks generally fall in the 15–25 MPG range.
US gallons (3.785 L). UK Imperial gallons are larger (4.546 L), so UK MPG figures are always higher than US MPG for the same vehicle. The L/100km figure is unambiguous.