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Fuel Economy Calculator

MPG, L/100km, and km/L — all three, from one tank.

L/100km and MPG measure the same thing in opposite directions — lower is better for L/100km, higher is better for MPG — which is why comparing cars across regions is confusing. This calculator takes whatever you actually measured from your last fill-up and gives you all four figures at once so you can compare apples to apples.

L/100km
7.00L/100km
km / L
14.29
MPG (US)
33.60
MPG (UK)
40.35

Typical range for most modern cars in mixed driving.

Why your real-world figure differs from the window sticker

Official economy ratings (EPA in the US, WLTP in Europe) are measured on a rolling road under controlled conditions: specific ambient temperature, no HVAC load, a prescribed drive cycle. They're designed for comparison, not prediction.

The real world is messier. Cold engines run rich for 10–15 minutes. Motorway speeds above 100 km/h (62 mph) increase aerodynamic drag exponentially — wind resistance scales with the cube of speed, so 120 km/h burns roughly 40% more than 90 km/h on a highway. Air conditioning on a hot day can add 1–2 L/100km by itself.

The five biggest factors affecting fuel economy

Most people focus on the car. The driver and conditions matter more.

  • Speed: highway driving above 110 km/h (68 mph) costs 15–30% more fuel than 90 km/h. The drag curve is exponential, not linear.
  • Tyre pressure: each 10 PSI under target adds ~0.5–1% fuel use from rolling resistance. Free fix — check monthly.
  • Cold starts: the first 5–10 minutes of a cold engine use 2–3× as much fuel per kilometre. Short trips destroy fuel economy averages.
  • Air conditioning: on small cars, A/C at full blast can consume 1.5–2 L/100km equivalent in extra fuel load.
  • Payload and drag: every 45 kg of extra weight costs ~1% in fuel economy. Roof racks and open windows add meaningful aerodynamic drag at speed.

How to improve your real-world MPG

The fastest return is tyre pressure — it takes 5 minutes and the effect is immediate. A correctly inflated tyre on a 1,500 kg car rolling at highway speeds cuts tyre-induced fuel loss by half compared to a 10-PSI underinflated tyre.

Second is driving style. Smooth acceleration and early gear changes (for manuals) use 10–15% less fuel than aggressive driving on the same route. In automatic cars, light throttle application triggers early upshifts into taller gears.

Third is trip consolidation. Warming up a cold engine for short errands burns a disproportionate amount of fuel. Combining a 5 km round-trip into one 15 km trip improves average economy significantly.

How to measure your true fuel economy accurately

The fill-up method is the most accurate way to track real consumption over time.

  • Fill completely at the same pump until the nozzle clicks off.
  • Reset your trip odometer to zero.
  • Drive normally — a longer measurement period gives a more accurate average.
  • Fill again to the click at the same pump if possible.
  • Record litres added and km driven and enter them above.
  • One tank is noisy. Average 3–5 tanks for a reliable baseline.

How to calculate your car's real fuel economy

Takes two numbers and a minute. Here's how to do it right.

  1. 1
    Record your starting odometer
    After a full fill-up, note the exact odometer reading (or reset the trip meter). This is your start point.
  2. 2
    Drive normally
    Longer samples are more accurate. A full tank (400–600 km) gives a much better average than a 100 km sample.
  3. 3
    Fill up again and record fuel added
    Fill completely until the pump clicks off. Note the litres (or gallons) added — this is your fuel consumption for the interval.
  4. 4
    Note distance covered
    Odometer end minus start, or read the trip meter directly.
  5. 5
    Enter into the calculator
    Select your units (km + litres, or miles + US gallons), enter distance and fuel used. All four economy figures appear instantly.

FAQ

How do you convert MPG to L/100km?
L/100km = 235.215 ÷ MPG (US). A car that does 30 MPG uses about 7.84 L/100km. This calculator does the conversion instantly — enter your distance and fuel used in whichever units you have.
What's the difference between US and UK gallons?
A UK gallon is about 20% larger than a US gallon (4.546 L vs 3.785 L). A car rated at 40 MPG (UK) is roughly equivalent to 33 MPG (US) — so UK figures always look better on paper.
What's a good fuel economy figure?
Under 5 L/100km (~47 MPG US) is excellent — hybrid or very efficient petrol territory. 6–9 L/100km is typical for modern petrol cars in mixed driving. Over 12 L/100km is thirsty unless you're driving a large petrol SUV or performance car.
Why does my actual MPG differ from the EPA/manufacturer rating?
Official ratings are tested on a dynamometer in controlled conditions. Real-world MPG is affected by driving style, speed, HVAC use, cold weather, tyre pressure, and payload. Most drivers see 10–20% below the official figure in mixed use.
How does tyre pressure affect fuel economy?
Underinflated tyres flex more under load, generating heat and rolling resistance. Each 10 PSI underinflation typically reduces fuel economy by 0.5–1%. Keeping tyres at the correct pressure is the cheapest fuel-saving modification available.
What is km/L and where is it used?
Kilometres per litre is the metric equivalent of miles per gallon — higher is better. It's the primary fuel economy unit in Japan, India, and parts of Southeast Asia. The inverse of km/L (×100) gives you L/100km, which is used in Europe and Australia.

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