Gas mileage and fuel-cost formulas
MPG: miles per gallon = miles driven ÷ gallons used
L/100km: litres per 100 km = 235.215 ÷ MPG
trip cost: cost = (distance ÷ MPG) × price per gallon
MPG and L/100 km describe the same thing inversely: a higher MPG is better, but a lower L/100 km is better. The conversion constant 235.215 comes from the litres-per-gallon and km-per-mile factors.
Worked example
You drove 320 miles on 12 gallons:
MPG: 320 ÷ 12 ≈ 26.7 mpg.
L/100km: 235.215 ÷ 26.7 ≈ 8.8 L/100km.
A 500-mile trip at $3.50/gal: (500 ÷ 26.7) × $3.50 ≈ $65.55.
Improving your mileage
Fuel economy drops sharply above about 50 mph, and aggressive acceleration and braking can cut it by 15–30% on the highway. Keeping tyres properly inflated, removing roof racks and excess weight, and steady cruising all help. The official figures behind US window stickers come from the EPA.
Tip: measure real-world MPG over several fill-ups rather than one. Fill the tank, reset the trip meter, and at the next fill divide the miles by the gallons it took to refill.