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Fuel economy guide

How to calculate gas mileage (MPG and L/100km).

Gas mileage, also called fuel economy or fuel efficiency, measures how far a vehicle can travel on a given amount of fuel. It is the primary metric for comparing the running cost of different vehicles, planning road trip budgets, and tracking whether a car's performance is declining over time (a drop in MPG can signal mechanical issues before the dashboard warning light appears).

The MPG formula (US imperial)

Miles per gallon (MPG) is the standard fuel economy metric in the United States. It tells you how many miles you drove for each gallon of fuel consumed:

Example: you drove 320 miles and used 11.2 gallons. MPG = 320 ÷ 11.2 = 28.6 MPG.

The higher the MPG, the more fuel-efficient the vehicle. A car that gets 40 MPG costs roughly half as much to fuel as one that gets 20 MPG over the same distance.

The L/100km formula (metric)

In Canada, Europe, Australia, and most countries outside the US, fuel economy is expressed in litres consumed per 100 kilometres. A lower number means better efficiency:

Example: you drove 480 km and used 36 litres. L/100km = (36 ÷ 480) × 100 = 7.5 L/100km.

Converting between MPG and L/100km

The exact conversion factor is based on the US gallon (3.785411784 L) and the statute mile (1.609344 km):

Examples: 28 MPG ≈ 8.4 L/100km · 10 L/100km ≈ 23.5 MPG. Note that the conversion is reciprocal — doubling MPG halves L/100km, not doubles it.

How to measure your fuel economy accurately

1. Fill up completely. Start with a full tank — top up until the pump clicks off automatically.
2. Reset the trip odometer to zero immediately after filling.
3. Drive normally for at least 200 miles (320 km) for a meaningful result. Short trips overstate fuel consumption because cold starts use extra fuel.
4. Fill up again at the same pump type (full-service vs self-serve, same octane). Note the litres or gallons pumped — this is your fuel used.
5. Read the odometer and subtract the reset value to get distance driven.
6. Calculate: miles ÷ gallons = MPG. Or enter both numbers in this calculator and get the result instantly.

Factors that affect fuel economy

Advertised MPG ratings (from the EPA or Transport Canada) are measured in controlled lab conditions. Real-world fuel economy typically runs 15–20% lower. The most impactful variables are:

• Driving speed: Aerodynamic drag increases with the square of speed. A car travelling at 80 mph uses roughly 25–30% more fuel than the same car at 60 mph.
• Acceleration and braking: Aggressive acceleration burns significantly more fuel. Smooth, anticipatory driving — coasting to red lights instead of braking hard — can improve fuel economy by 10–40% in city driving.
• Tyre pressure: Under-inflated tyres increase rolling resistance. Each 1 PSI drop in tyre pressure reduces MPG by about 0.2%. Keeping tyres at the recommended pressure is free and effective.
• Air conditioning: A/C can reduce fuel economy by 5–25% depending on outside temperature, vehicle size, and driving speed. At highway speeds, windows-up + A/C is often more efficient than windows-down (which increases drag).
• Payload and roof racks: Extra weight (passengers, cargo) and wind-catching accessories like roof boxes directly increase fuel consumption. A 100 lb (45 kg) increase in payload reduces MPG by about 1–2% for a typical car.
• Engine and drivetrain health: Dirty air filters, worn spark plugs, and incorrect ignition timing can reduce fuel economy by 10–20%. Regular maintenance keeps efficiency at its rated level.
• Fuel grade: Using a higher-octane fuel than the manufacturer specifies provides no efficiency benefit for most engines. Engines designed for premium fuel will suffer in efficiency if run on regular.
• Cold weather: Cold engines use more fuel during warm-up. Engine oil is thicker in cold weather, increasing friction. At –7°C (20°F), short-trip MPG can be 25–35% worse than in warm weather.

Tips to improve your gas mileage

• Maintain a steady speed on the highway — use cruise control.
• Anticipate traffic and decelerate gently rather than braking hard.
• Reduce idling: a modern engine uses ~0.2–0.6 gallons per hour at idle.
• Check and maintain proper tyre inflation monthly.
• Remove unnecessary weight from the car (tools, sports gear).
• Plan errands to combine multiple trips into one warm-engine journey.
• Service the engine at manufacturer-recommended intervals.
• Use the manufacturer's recommended grade of motor oil.

Annual fuel cost calculation

Once you know your MPG and gas price, annual fuel cost is:

For metric: Annual cost = (Annual km ÷ 100) × L/100km × Price per litre.

Example: 28 MPG, 15,000 miles/year, $3.50/gallon: 15,000 ÷ 28 × $3.50 = $1,875/year.

This calculation is useful for comparing the true ownership cost of two vehicles. A car with a $3,000 higher purchase price but 10 MPG better fuel economy can save money over 5 years — use the annual cost formula to run the numbers.

Disclaimer

Results are calculated from the values you enter. Fuel economy varies with driving conditions, vehicle maintenance, and fuel quality. For vehicle-specific ratings, consult the EPA's fueleconomy.gov (US) or Natural Resources Canada's nrcan.gc.ca/transportation (Canada).