We get a number of e-mails from readers each week complaining about their vehicle's fuel consumption and we thought we would answer the question posed by reader Tanya. Tanya asked how the fuel consumption figure on the vehicle's label is determined and why her car uses more fuel than the figure on the label.
This article was first published in September, 2015
Q: I bought a Toyota LandCruiser Sahara a few months ago and I'm finding that I can never get close to the fuel consumption figure listed on the windscreen sticker. Is there something wrong with my car? Why can Toyota claim the car is using less fuel than it actually does? How do they come up with these figures?
A: Great question, Tanya. The figures listed on a car's windscreen sticker are derived quite precisely and each vehicle is tested using the same testing procedure to ensure each vehicle runs to the same benchmark.
The testing procedure is detailed under ADR 81/02 Fuel Consumption Labelling for Light Vehicles. The procedure is conducted over a continuous period of 20 minutes and is split into an urban and extra-urban cycle.
Urban cycle testing accounts for around 67 per cent of the test, while the extra-urban cycle accounts for the remaining 33 per cent.
The urban cycle consists of a lower average speed and is designed to simulate driving in a city environment with a constant element of stop/start driving at an average speed of 19km/h and considerable idle periods of around 30 per cent.
The extra-urban cycle is run at a higher average speed of 63km/h with a peak speed of 120km/h. It's not an accurate representation of the regular fuel consumption on a highway, as the test isn't conducted as a set speed over a period of time.
Below is a graph that details the testing procedure to give you an idea of the time spent at each speed.
Beyond the standard information above, some interesting tidbits from ADR 81/02:
- All vehicles tested need to have zero kilometres on the odometer. If the manufacturer wants to run in a vehicle prior to testing (up to 15,000km), a test must be done at 0km, along with the final km reading on the vehicle. These two numbers are then divided to achieve an 'evolution coefficient', which is then used to calculate the final emissions figure.
- A number of readings are taken to determine the fuel consumption outside of just the figure read by the vehicle. The density of the fuel used (measured at 15 degrees Celsius), along with the emission of hydrocarbons, carbon monoxide and carbon dioxide are pooled together in a calculation to determine fuel consumption.
- Tyre pressures, engine oil viscosity and all other consumables are to be at recommended manufacturer levels.
- All lights and signalling devices should be switched off or in the vehicle's normal operating state.
- All testing is conducted between 20 and 30 degrees Celsius ambient temperature.
With that in mind, when we introduce things like wind, extra weight, towing, air conditioning and any other number of variables into the equation, the variation from the manufacturer claims can begin to vary.
We find more often than not when testing vehicles that it's hard to meet the claimed fuel consumption figures. That's because the claimed figures are recorded in ideal conditions. When a car is put into real world conditions, these aspects change dramatically.
Another thing to consider that your driving style has a lot to do with the vehicle's fuel economy. Braking late and accelerating hard will reduce component life and will cause fuel consumption to increase.
Tanya, with your LandCruiser specifically, you need to take into account that it's a heavy vehicle and added extra weight by towing or carrying passengers will affect its fuel consumption. Anything beyond a 50+ per cent variation needs to be checked out by the dealership.