How do we calculate this?
Everything runs in your browser. We first convert everything to metric units, adjust consumption with factors from government agencies, national laboratories and motoring clubs, and then convert the result back to your units.
- Total distance = distance × 2 for a return trip × number of trips.
- Real-world baseline = the figure you enter, corrected if it is an official one (WLTP, NEDC or EPA).
- We split the distance into town, country road and motorway, and work out each part’s consumption with the weight, aerodynamics and air-conditioning factors.
- With traffic jams, a limited part of the trip is calculated as slow traffic (replacing, not adding to, the rest).
- Litres = total distance × adjusted consumption / 100. Cost = litres × price per litre.
How the factors combine
Road, weight and aerodynamic factors are multiplied, because each one scales the energy needed to move the car. Air conditioning is added as a per-hour consumption. To avoid counting anything twice, the road factors are scaled so that typical use leaves the result unchanged; that way they do not overlap with the official-to-real-world gap. Traffic-jam kilometres also replace other kilometres instead of being added on top.
Estimate range
Each factor has a minimum, central and maximum value based on the evidence. We work out the effect of each extreme separately and combine them as the root of the sum of squares, because all factors are unlikely to be off in the same direction at once.
Factor values
| Figure | Petrol | Diesel |
|---|---|---|
| Official WLTP | +20%+12% – +25% | +17%+12% – +22% |
| Official NEDC | +39%+30% – +45% | +39%+30% – +45% |
| Official EPA | +2%0% – +10% | +2%0% – +10% |
| Factor | Town | Country road | Motorway |
|---|---|---|---|
| Road type (normalised) | ×1.07×1.00 – ×1.20 | ×0.84×0.78 – ×0.91 | ×1.10×1.00 – ×1.24 |
| Fuel/weight elasticity | 0.450.35 – 0.55 | 0.350.25 – 0.45 | 0.250.15 – 0.35 |
| Windows down (car) | +1%0% – +2% | +4%+2% – +6% | +7%+4% – +8.5% |
| Windows down (SUV/van) | +0.5%0% – +1% | +2%+1% – +3% | +3%+1% – +4% |
| Roof box (car) | +4%+2% – +8% | +10%+6% – +17% | +17%+10% – +25% |
| Air conditioning, car in hot weather (L/100 km) | 2.21.4 – 3 | 0.80.5 – 1.1 | 0.50.3 – 0.7 |
Vehicle types
The vehicle type does not replace your fuel economy figure: it adjusts how weight, speed, aerodynamics and air conditioning affect it. The adjustments come from a simple physical model (aerodynamic drag Cd·A, rolling resistance and mass of each type), calibrated with ORNL data for cars and with real-world measurements by promobil for motorhomes.
| Type | Mass in running order | Travel equipment | Motorway | WLTP gap (diesel) | A/C in hot weather (L/h) |
|---|---|---|---|---|---|
| Not specified | 1,450 kg | – | ×1.00 | +17%+12% – +22% | 0.70.45 – 1 |
| Car | 1,350 kg | – | ×1.00 | +17%+12% – +22% | 0.550.35 – 0.75 |
| SUV | 1,600 kg | – | ×1.04×1.00 – ×1.08 | +17%+12% – +22% | 1.10.7 – 1.5 |
| Van | 2,000 kg | – | ×1.05×1.00 – ×1.10 | +8%0% – +17% | 1.20.8 – 1.6 |
| Campervan | 2,650 kg | +100 kg | ×1.05×0.95 – ×1.10 | +5%-5% – +15% | 1.30.9 – 1.7 |
| Motorhome | 3,000 kg | +200 kg | ×1.15×1.00 – ×1.30 | 0%-10% – +10% | 1.41 – 1.8 |
Not specified
Average of the petrol and diesel car fleet. New cars in the EU weigh 1,554 kg on average (2024, including electric cars), but the fleet is 12.7 years old on average and lighter.
Car
Any car that is not an SUV: superminis, hatchbacks, saloons, estates and MPVs. Mass of 1,350 kg and Cd·A of 0.75 m², from ICCT European averages. It is the reference for the other adjustments.
SUV
SUVs and crossovers. They are heavier and have more aerodynamic drag (Cd·A ≈ 0.98 m², ICCT), so motorways cost them a little more and open windows a little less in proportion (ORNL).
Van
Passenger or cargo vans, from the Berlingo to the Multivan. With Cd·A ≈ 1.2 m² they suffer more on motorways. Their WLTP figure is closer to real-world use than a car’s: the Multivan T6.1 is rated at 8.2 L and averages 8.65 L on Spritmonitor. ⚠️ Estimate to be reviewed.
Campervan
Van conversion with a pop-top or fixed high roof (VW California, Ford Nugget, Mercedes Marco Polo, converted Fiat Ducato or Citroën Jumper). We add 100 kg of travel equipment (water, gas, gear). Its WLTP figure is very close to real-world use (+5%). ⚠️ Estimate to be reviewed.
Motorhome
Semi-integrated or coachbuilt motorhome on a Fiat Ducato or similar chassis, with a 3,500 kg gross weight. It weighs about 3,000 kg in running order and we add 200 kg of travel equipment, as the ADAC does; with two people and luggage it is close to the gross weight. We estimate a Cd·A of 2.8 m², which reproduces the +28% promobil measured going from 85 to 105 km/h. Its WLTP figure matches real-world use on average. ⚠️ Estimate to be reviewed.
Other parameters
- Weight per extra traveller: 75 kg. Light luggage 50 kg, heavy 150 kg, roof box 50 kg.
- Air conditioning in hot weather: 0.55 L/h (car) to 1.4 L/h (motorhome). Mild weather ×0.35; warm ×0.65. Diesel ×0.85.
- Moderate traffic: 15% of the leg (max. 10 km) at ×1.25 of town consumption. Heavy: 30% (max. 25 km) at ×1.8.
- Automatic route: sections below 50 km/h count as town, 50–85 km/h as country road and above 85 km/h as motorway (speeds estimated by OSRM).
Factors and sources
Official → real-world
Official figures come from a lab. Using real data from millions of cars, the EU and the ICCT find that cars on the road use on average 17–20% more than WLTP, 39% more than NEDC and 2% more than the EPA label.
Road type
The official figure is a mixed average. Town driving uses more because of stops, country roads at moderate speed use less, and motorways use more because air drag grows with the square of speed.
Passengers and luggage
Each extra kilo takes more energy to accelerate and roll. The effect depends on the car’s weight (100 kg matters more in a city car than in a motorhome) and is larger in town. We subtract the load already included in the official test.
Windows down
Open windows increase aerodynamic drag. In town you barely notice it, but on the motorway it can cost up to 8% in a car and 4% in an SUV (ORNL).
Roof rack or box
A roof box hurts aerodynamics a lot: 2–8% in town and 10–25% on the motorway. ADAC measured 18% more at 130 km/h.
Air conditioning
The compressor uses an almost fixed amount per hour (≈0.5–1.5 L/h in hot weather, according to ORNL). At low speed that is spread over few kilometres and weighs a lot; on the motorway it barely shows.
Traffic jams
Constant stopping and starting drives up consumption per km: at an average of 10 km/h a car uses almost twice as much as in flowing town traffic. We only apply it to a limited part of the trip.
Limitations
- How air conditioning scales with temperature, its diesel adjustment and the length of traffic jams are reasoned estimates still to be reviewed.
- The van, campervan and motorhome adjustments rely on few models and on a physical model; the motorhome Cd·A is an estimate, because manufacturers do not publish it.
- We do not model plug-in hybrids, whose consumption depends on how much they are charged, or electric cars.
- Driving style can change consumption by 10–40% (fueleconomy.gov). If you know your real figure, use it.