Gas Mileage (MPG) Calculator

How to use this tool

1. Select your unit system. Choose US customary (miles, US gallons) or Metric (km, liters) to match your odometer and fuel gauge.
2. Choose how to enter distance. Enter the trip distance directly, or choose odometer readings and enter the start and end values. The end reading must be greater than the start reading.
3. Enter fuel and optional price. Enter the amount of fuel used in gallons or liters. Add your pump price per unit to estimate the fill-up cost and cost per distance.
4. Read all three metrics. The calculator instantly shows MPG, km/L, and L/100km. The optional cost estimate uses only the pump price you enter.

How to calculate MPG

Fill your tank completely, then drive a known distance. When you refill, note how many gallons it took. MPG = miles driven ÷ gallons used. For example, 300 miles on 10 gallons = 30 MPG.

Common fuel economy conversions

The US option uses US gallons (3.785 liters per gallon), not UK Imperial gallons (4.546 liters per gallon). Choose the unit that matches your fuel records. When you enter a pump price, the calculator multiplies it by the fuel used for this fill-up and divides by distance for a cost per mile or kilometer. The selected currency controls the amount’s display format; no exchange rate is applied.

MPG vs L/100km explained

MPG (miles per gallon) is used in the United States. A higher number means better fuel economy. L/100km (liters per 100 km) is used in most of the world; a lower number is more efficient. The two are inversely proportional: L/100km = 235.214 ÷ MPG.

Factors affecting fuel economy

• Speed: most vehicles achieve peak fuel economy between 45–55 mph. Above 60 mph, aerodynamic drag increases sharply - driving at 80 mph can reduce economy by 20–30%.
• Tire pressure: under-inflated tires increase rolling resistance. Maintaining proper pressure can improve MPG by 0.5–3%.
• Cargo and roof racks: every 100 lbs of extra weight reduces MPG by approximately 1%. An empty roof rack increases drag by 2–8%.
• Air conditioning: AC can reduce fuel economy by 5–25% in city driving, less at highway speeds.
• Cold starts: engines run rich when cold, burning up to 50% more fuel for the first 1–2 miles.

Annual fuel cost calculation

Estimated annual fuel cost = (annual miles ÷ MPG) × price per gallon. At 15,000 miles/year, 30 MPG, and $3.50/gal: 15,000 ÷ 30 × $3.50 = $1,750/year. Improving to 40 MPG cuts this to $1,313 - a $437 annual saving.

MPGe for electric vehicles

The US EPA defines Miles Per Gallon equivalent (MPGe) as the number of miles an EV travels on 33.7 kWh of electricity - the energy content of one gallon of gasoline. An EV rated at 100 MPGe uses 33.7 kWh to travel 100 miles. MPGe allows direct cost comparisons between EVs and conventional vehicles.