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 | +4%+2% – +8% | +10%+6% – +17% | +17%+10% – +25% |
| Air conditioning, compact in hot weather (L/100 km) | 2.21.4 – 3 | 0.80.5 – 1.1 | 0.50.3 – 0.7 |
Other parameters
- Weight per extra traveller: 75 kg. Light luggage 50 kg, heavy 150 kg, roof box 50 kg.
- Air conditioning in hot weather: 0.5 L/h (small car) to 1.2 L/h (van). 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 an SUV) 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.
- 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.