Fuel Consumption Calculator - L/100km, MPG and km/L

Same car, same road - 6 L/100km in summer, 9 in winter

Your car's official WLTP figure says 5.8 L/100km. Real life disagrees. Cold starts, air conditioning, roof boxes, tire pressure and driving style can push real consumption 30-50% higher than the lab number. This calculator works from your actual fill-up data - liters in, kilometers driven, honest result.

How to measure fuel consumption - step by step

Fuel added (liters) - fill up to the nozzle click at the pump. Write down the exact number of liters. Always fill to the same point for consistency.
Distance driven (km) - enter the kilometers since the last full fill-up. Use the trip meter or subtract odometer readings. Minimum 300 km for a reliable average.
Vehicle type - optionally select your car category. The calculator will rate your consumption as efficient, average or high compared to typical values for that class.
Read the results - you get L/100km, US MPG, UK MPG, km/L and a color-coded rating.

Four key numbers at a glance

30-50%

Gap between WLTP lab figures and real-world consumption

25%

Extra fuel burned at 85 mph vs 70 mph due to air resistance

0.6 psi

Tire pressure drop per month - each 1 psi low adds 0.2% to consumption

15%

Extra consumption from running AC at full blast in city traffic

Consumption benchmarks by vehicle type

Vehicle type	Efficient	Average	High	US MPG equiv.
City car	< 5.5	5.5-7.0	> 7.0	34-43
Compact	< 6.5	6.5-8.0	> 8.0	29-36
Sedan/Estate	< 7.0	7.0-9.0	> 9.0	26-34
Compact SUV	< 8.0	8.0-10.0	> 10.0	24-29
Large SUV	< 10.0	10.0-13.0	> 13.0	18-24
Hybrid	< 5.0	5.0-6.0	> 6.0	39-47
Sports car	< 11.0	11.0-16.0	> 16.0	15-21

Did you know?

Cold engine penalty: The first 5 km after a cold start burn 40-80% more fuel than a warm engine. Short city trips are the worst case for fuel economy.

Roof box drag: An empty roof box adds 0.5-1.0 L/100km at highway speeds. Remove it when not in use - aerodynamic drag scales with the square of speed.

MPG confusion: US gallon = 3.785 L, UK gallon = 4.546 L. A car rated 30 MPG (US) is 36 MPG (UK). Always check which gallon the spec uses.

Weight matters: Every extra 100 kg of cargo adds roughly 0.3-0.5 L/100km. Clean out the trunk before a long trip.

Diesel vs petrol: Diesel engines are typically 15-25% more efficient in L/100km, but diesel fuel contains 13% more energy per liter. The real CO2 difference is smaller than the consumption gap suggests.

Practical examples

Daily commute (compact): 38 liters used over 520 km of mixed city/highway driving.
Result: 7.31 L/100km (32.2 MPG US) - average for a compact car.

Highway road trip (sedan): 42 liters over 680 km of steady motorway cruising.
Result: 6.18 L/100km (38.1 MPG US) - very efficient for a 2.0L sedan on the highway.

Winter city driving (SUV): 55 liters over 420 km with cold starts and short trips.
Result: 13.10 L/100km (18.0 MPG US) - high for a compact SUV. Winter penalty + city driving.

Hybrid in city traffic: 28 liters over 620 km of stop-and-go commuting.
Result: 4.52 L/100km (52.1 MPG US) - hybrids shine in exactly this scenario.

Sports car, spirited driving: 65 liters over 380 km of back roads and overtaking.
Result: 17.11 L/100km (13.8 MPG US) - expected for a sports car driven enthusiastically.

Unit conversion reference

L/100km	MPG (US)	MPG (UK)	km/L
5.0	47.0	56.5	20.0
7.0	33.6	40.4	14.3
9.0	26.1	31.4	11.1
12.0	19.6	23.5	8.3

FAQ - Frequently asked questions

How do I convert L/100km to MPG?

Divide 235.214 by your L/100km figure for US MPG, or 282.481 for UK MPG. Example: 8 L/100km = 235.214 / 8 = 29.4 MPG (US). The calculator does this automatically.

Why is my real consumption higher than the official WLTP figure?

WLTP tests run in a controlled lab at 23 degrees C, with no AC, no wind, no payload, and a standardized driving cycle. Real conditions add cold starts, elevation changes, traffic, weather and personal driving style. A 30-50% gap is typical, and some cars deviate even more.

How accurate is the fill-up method?

Very accurate if done consistently. Always fill to the same point (nozzle auto-click), measure over at least 300 km, and avoid measuring after a single city-only or highway-only tank. Three consecutive fill-ups averaged together give a reliable baseline.

What is the difference between US and UK MPG?

A US gallon is 3.785 liters; a UK (imperial) gallon is 4.546 liters. The UK gallon is about 20% larger. So the same car shows 30 MPG (US) = 36 MPG (UK). Always check which gallon a specification uses before comparing.

Does driving faster always use more fuel?

Yes, above about 50-60 km/h. Aerodynamic drag rises with the square of speed - doubling your speed quadruples air resistance. Going from 110 to 140 km/h can increase consumption by 20-30%. The sweet spot for most cars is between 80-100 km/h in the highest gear.

Can I use this calculator for diesel and hybrid cars?

Yes. The formula L/100km = (liters / km) x 100 works the same regardless of fuel type. Select the matching vehicle type for an appropriate consumption rating. For plug-in hybrids, the result reflects the fuel portion only - electric energy consumed is not included.

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