The math
Fuel economy math is simple division. The complication is just unit conversion: MPG goes up as efficiency improves, L/100km goes down. They measure the same thing inversely.
mpg = miles ÷ gallons
L/100km = (liters ÷ km) × 100
conversion: 235.215 ÷ mpg = L/100km
Frequently asked
How can I improve real-world MPG?
The biggest gains come from driving style: gentler acceleration, easing off well before stops, and keeping highway speeds under 70 mph. Properly inflated tires save 3-5%. A clean air filter and oxygen sensor matter. Roof racks and roof boxes can cost 5-15% at highway speeds.
Is it worth buying a more efficient car?
Depends on the price gap and how much you drive. Use the comparison mode here to project savings over a year, then compare to the price difference. At $3.50/gallon and 12,000 miles/year, switching from 22 to 42 mpg saves about $900/year. Whether that justifies a $10,000 price difference depends on how long you keep cars.
What about EVs?
EVs are measured in MPGe (miles-per-gallon equivalent) or kWh/100mi. To compare costs, convert to dollars: a 30 kWh/100mi EV at $0.13/kWh costs about $3.90 per 100 miles. A 30 mpg car at $3.50/gallon costs about $11.67 per 100 miles. EVs win on fuel cost almost everywhere.