Calculate Your Real Fuel Economy & Trip Costs
Convert between MPG, L/100km and Km/L, estimate gas costs, and project your monthly and annual fuel spending — built for drivers in the USA, UK, Canada and Europe.
Benchmark uses a representative efficient petrol car for your selected region and unit system. Actual savings vary with driving style, maintenance and fuel prices.
How to Calculate Fuel Economy and Trip Gas Costs Accurately
Fuel economy is the distance a vehicle travels per unit of fuel, and you calculate it by dividing the distance traveled by the amount of fuel consumed. Whether you drive in the United States, the United Kingdom, Canada or mainland Europe, the underlying math is identical — only the display units differ. The calculator above performs this calculation instantly and converts the result into every common global unit so you can compare vehicles, plan trips and forecast budgets with confidence.
To get an accurate real-world reading rather than a manufacturer estimate, follow a disciplined measurement routine:
- Fill the tank completely at the start of your measurement period and reset your trip odometer to zero.
- Drive normally for the period you want to measure — a full tank cycle gives the most reliable figure.
- Refill the tank to full again and record exactly how much fuel the pump dispenses, plus the trip odometer reading.
- Divide distance by fuel used (for MPG/Km/L) or fuel used by distance × 100 (for L/100km).
For trip cost, multiply the fuel consumed by the pump price per gallon or liter. To project annual expenditure, scale your single measurement by the appropriate frequency multiplier: daily × 365, weekly × 52, monthly × 12. The calculator handles every conversion and projection automatically, removing the risk of manual arithmetic errors.
- Always measure from full tank to full tank for the most accurate figure.
- Trip cost = fuel used × price per unit; annual cost = period cost × frequency.
- Manufacturer (WLTP/EPA) figures are typically 10–25% higher than real-world results.
Understanding MPG vs L/100km: Conversions and Regional Differences (USA, UK, EU, Canada)
MPG and L/100km are two ways of expressing the same fuel economy, but they use inverted logic: MPG is distance-per-fuel (higher is better) while L/100km is fuel-per-distance (lower is better). Confusing the two is the most common source of comparison errors, especially because the United States and the United Kingdom both use "MPG" but define a "gallon" differently.
- USA — Miles per US gallon (1 US gal = 3.785 L).
- United Kingdom — Miles per Imperial gallon (1 UK gal = 4.546 L).
- Canada & Europe — Liters per 100 km (L/100km).
The UK Imperial gallon is about 20% larger than the US gallon, so a UK MPG figure will always read roughly 20% higher than the identical car measured in US MPG. This is why a vehicle advertised at "50 MPG" in the UK looks far thirstier when quoted in US terms — the car has not changed, only the gallon has.
Use these exact manual conversion factors:
- US MPG → L/100km = 235.215 ÷ US MPG
- UK MPG → L/100km = 282.481 ÷ UK MPG
- L/100km → US MPG = 235.215 ÷ L/100km
- L/100km → Km/L = 100 ÷ L/100km
- US MPG → Km/L = US MPG × 0.4251
- UK MPG → US MPG = UK MPG × 0.8327
For example, a car returning 30 US MPG equals 7.84 L/100km, 12.75 Km/L and about 36 UK MPG. The calculator above applies every one of these conversions in real time so you never need to remember the constants.
Proven Strategies to Improve Vehicle Fuel Efficiency and Reduce Driving Expenses
The fastest way to cut fuel costs is to change driving habits, because smooth, anticipatory driving alone can improve fuel economy by 15–30% at zero financial cost. Combined with light maintenance and smart route planning, most drivers can meaningfully reduce their monthly and annual fuel expenditure without buying a new vehicle.
- Smooth acceleration & braking — aggressive driving can lower MPG by 15–30% on highways and 10–40% in stop-and-go traffic.
- Observe the speed limit — each 5 mph above 50 mph adds roughly $0.20–$0.40 per gallon in effective cost.
- Remove excess weight & roof cargo — an extra 100 lbs typically cuts economy by about 1%; roof boxes hurt aerodynamics at speed far more.
- Keep tires at correct pressure — under-inflation by 1 psi can reduce economy by ~0.2%; correct pressure also improves safety.
- Use the recommended oil grade and replace air filters as specified to keep the engine breathing efficiently.
- Minimize idling — modern engines use less fuel restarting than idling for more than ~10 seconds; use stop-start if fitted.
- Combine trips — a warm engine is efficient, so chained errands beat many cold separate starts.
Over a full year these small adjustments compound dramatically. A driver covering 12,000 miles a year at 25 MPG who improves to 30 MPG saves roughly 80 gallons annually — at typical pump prices that is hundreds of dollars, pounds or euros back in the household budget. The savings comparison card above shows exactly how much you could reclaim versus an efficient benchmark vehicle.