Does cruise control actually save fuel?
Cruise control can save 7–14% fuel on a flat highway. On a hilly road, it can cost you more. Here's when to use it and when to turn it off.
Windows down or air con on: what uses less fuel?
Most Australians pick based on how hot it is, or whether they like the wind noise. Both are fine reasons, but neither one is the efficient choice. The real answer isn't about personal preference or driving habit. It depends on your speed, the shape of your car, and how hard the air con is actually working. Here's what the physics says and the practical rule you can use straight away.
How air con affects your fuel consumption
Your car's air conditioning system runs off the engine. To cool the cabin, it drives a compressor, and that compressor puts an extra load on the engine, which needs more fuel to maintain its output. On a hot summer day with the air con working hard, this load can add up to around 10% to your fuel consumption. On a mild day with the system on a low setting, the impact is noticeably smaller.
The penalty is consistent regardless of speed. Whether you're crawling through Sydney traffic or cruising at 100 km/h, the compressor is drawing similar power from the engine. The faster you drive, the relatively smaller this penalty becomes, because your total fuel consumption per kilometre drops at higher speeds, meaning the air con represents a smaller slice of the total.
How open windows affect your fuel consumption
Open windows create aerodynamic drag. At low speeds, this is negligible. The extra turbulence created by an open window at 40 km/h barely touches your fuel economy. But as speed increases, aerodynamic drag scales up dramatically. The relationship is not linear: drag force roughly quadruples when you double your speed. By the time you're on a freeway, open windows can increase fuel consumption by as much as 20% for some vehicle shapes, particularly sedans and hatchbacks where the open window creates significant turbulence at the roofline. A on the freeway with all four windows open, for example, pays a meaningfully higher drag penalty than a taller in the same conditions.
