Fuel Consumption Calculation: km/l, l/100km and Trip Cost [Guide]

How much fuel does your car really consume? And how much will your next trip cost in terms of fuel? Fuel consumption is one of the most significant expenses for Italian motorists, yet many do not know how to calculate it precisely. In this complete guide, you will learn how to measure your car's real consumption using the full-tank-to-full-tank method, the difference between km/l and l/100km, the factors that influence consumption, and the best tips for saving at the pump.

km/l and l/100km: two ways to express consumption

Fuel consumption can be expressed in two main ways, and it is important to understand the difference:

Kilometers per liter (km/l)

Indicates how many kilometers you travel with one liter of fuel. The higher the number, the better. It is the most intuitive format for many Italians.

Example: 15 km/l means you travel 15 km with one liter.

Liters per 100 kilometers (l/100km)

Indicates how many liters you consume to travel 100 km. The lower the number, the better. It is the format used by European car manufacturers in technical specifications and the most common in official tests.

Example: 6.5 l/100km means 6.5 liters are needed to travel 100 km.

Conversion between the two formats

Example: a car that gets 15 km/l consumes 100/15 = 6.67 l/100km. A car that consumes 8 l/100km travels 100/8 = 12.5 km/l.

Quick conversion table

The full-tank-to-full-tank method: how to measure real consumption

Data declared by car manufacturers is based on standardized test cycles (WLTP in Europe) that often do not reflect real-world consumption. To measure your car's actual consumption, use the full-tank-to-full-tank method.

Step-by-step procedure

1. Fill up completely at the gas station (until the nozzle clicks off).
2. Reset the trip meter or note the total odometer reading.
3. Drive normally for at least 300-500 km (the more kilometers you drive, the more accurate the result will be).
4. Fill up completely again at the same gas station, preferably at the same pump.
5. Note the liters dispensed at the second fill-up and the km traveled.

Calculation

Consumption (km/l) = Kilometers traveled / Liters at second fill-up

Consumption (l/100km) = (Liters at second fill-up / Kilometers traveled) x 100

Practical example

You traveled 487 km and put in 34.2 liters at the second fill-up.

Tips for accurate measurement

• Always use the same gas station and the same pump
• Fill up under the same conditions (car on level ground, nozzle clicks off)
• Travel at least 300 km to reduce measurement error
• Repeat the measurement 2-3 times and take the average
• Measure city, highway, and mixed consumption separately

Calculating trip fuel cost

Once you know your car's consumption, calculating the fuel cost of a trip is straightforward.

Formula

Trip cost = (Distance in km / km per liter) x Price per liter

Or: Trip cost = (Distance / 100) x l/100km x Price per liter

Trip cost examples (average prices March 2026)

Note: the costs shown refer to fuel only. For a complete trip, highway tolls, stops, and maintenance should also be considered.

Factors that influence fuel consumption

A car's real consumption depends on many factors. Knowing them helps you understand why your car might consume more (or less) than declared.

Driving-related factors

• Driving style: harsh acceleration and sudden braking can increase consumption by 20-30%
• Speed: at 130 km/h, consumption is approximately 20-25% higher than at 110 km/h due to aerodynamic drag
• Gear selection: driving at too low or too high RPMs increases consumption
• Air conditioning use: A/C can increase consumption by 5-15%

Vehicle-related factors

• Tire pressure: tires underinflated by 20% increase consumption by 2-3%
• Weight carried: every extra 100 kg increases consumption by 3-5%
• Roof rack and roof box: increase aerodynamic drag, up to +15% consumption
• Engine condition: dirty filters, worn spark plugs, and old oil reduce efficiency
• Fuel type: using the manufacturer-recommended fuel optimizes consumption

Environmental factors

• Route: city driving (traffic, traffic lights) consumes much more than highway or rural roads
• Gradient: uphills and downhills affect consumption (uphills increase it significantly)
• Temperature: cold weather increases consumption (colder engine, heating, oil resistance)
• Wind: driving into a headwind increases drag and therefore consumption

Average consumption by vehicle type in Italy

Practical tips for saving fuel

1. Drive smoothly and anticipate

Accelerate gradually, maintain a constant speed, and anticipate braking by releasing the accelerator in time. This single habit can reduce consumption by 15-20%.

2. Check tire pressure

Check pressure at least once a month and before long trips. Consult your owner's manual for the correct values. Properly inflated tires also improve safety.

3. Reduce unnecessary weight

Remove everything you don't need from the trunk. Remove roof racks and roof boxes when not in use. Every kilogram less means fuel saved.

4. Use air conditioning wisely

At low speeds, lower the window. On the highway, close the windows and use the A/C (open windows at high speed create more drag than the A/C). Set the temperature to 22-24°C, not the minimum.

5. Plan your routes

Use navigation to avoid traffic. Group errands into a single route. A warm engine consumes less than multiple cold starts.

6. Choose the right gas station

Fuel prices can vary by 10-15 cents per liter between nearby stations. On a 50-liter fill-up, the difference can exceed 7 euros. Use apps like Prezzi Benzina or Fuelio to find the most affordable stations.

7. Consider alternatives

For short, repetitive commutes (home-work), consider LPG, natural gas, hybrid, or electric. For occasional long trips, car sharing or the train may be more economical.

Gasoline, diesel, LPG, natural gas: cost per km comparison

Note: prices are indicative and vary by area, station, and time period. Typical consumption refers to a C-segment car (such as a Volkswagen Golf).

Frequently asked questions (FAQ)

How do I calculate my car's fuel consumption?

Use the full-tank-to-full-tank method: fill up, note the km, fill up again, and divide the km traveled by the liters dispensed at the second fill-up. You get the consumption in km/l.

What is the difference between km/l and l/100km?

They are the inverse of each other. km/l indicates how many km you travel with one liter (higher is better). l/100km indicates how many liters are needed for 100 km (lower is better). Formula: l/100km = 100 / km/l.

Why does my car consume more than the declared figure?

Manufacturer WLTP data is based on standardized tests under optimal conditions. Real-world consumption is affected by driving style, traffic, air conditioning, weight, tire pressure, and weather conditions.

How much does a Rome-Milan car trip cost?

With a gasoline car consuming 7 l/100km, for the approximately 575 km you need about 40 liters, equal to approximately 70-72 euros of gasoline at current average prices. Highway toll (approximately 45-50 euros) should be added to this.

Is diesel or gasoline more economical?

Diesel is more economical if you drive more than 15,000-20,000 km per year, thanks to lower consumption and slightly lower fuel prices. For low mileage, gasoline has lower purchase and maintenance costs.

How can I reduce fuel consumption?

Drive smoothly, keep tires at the correct pressure, reduce unnecessary weight, use air conditioning wisely, and plan routes to avoid traffic. These habits can reduce consumption by 15-25%.

Conclusion

Knowing your car's real consumption is the first step toward saving on fuel and planning trips with awareness. Whether you need to calculate the cost of a trip, compare different cars, or simply understand whether your car consumes too much, the formulas and tips in this guide will be useful. Use our fuel consumption calculator to instantly get consumption in km/l and l/100km, cost per kilometer, and the total cost of any trip.