Saving fuel is not just about driving more slowly. The real cost comes at above 120km/h. Your engine may not sound like it’s labouring, but in reality that’s when your engine starts gulping fuel. At very high speeds engines will rev higher and will no longer be in the economical 2,000 to 2,500 rpm band. At this point the engine will have to work much harder.
Driving at 120km/h uses up to 9% more fuel than at 90km/h and up to 15% more than at 80km/h. 125km/h can use up to 25% more than at 120km/h.
Over time, tyres will naturally leak a bit of air. You can improve fuel consumption by up to 2% if you check pressures regularly, and keep them at the correct pressure. It’s safer, too – dozens of people a year die because of poorly-inflated tyres.
Some vehicle test reports say get rid of the rack – fully loaded it can add 30% to fuel consumption. But AutoExpress says it’s a myth – its test found that those with aerodynamic designs added little to the fuel bill.
Air con vs open windows
At low speeds, such as driving around town, air conditioning can add 5%-7% to fuel costs, so better to just wind the window down. But at higher speeds, the effects are less noticeable. However, when driving fast, an open window or sun roof increases aerodynamic drag and can add a further 3-6% to fuel costs.
In a survey by BP, 10% of drivers thought that turning off the radio improved fuel consumption. It doesn’t.
Don’t assume smaller engines use less petrol. A big car with a 2.0L engine may use less than the same car with a 1.6L engine as it strains to pull a larger weight. In vehicle tests, a heavy car with a small engine records much worse fuel efficiency than one with a bigger engine.
Petrol vs diesel
In general, diesels will use less fuel, but, as they typically cost more to buy, the savings aren’t always obvious. The Environmental Transport Association has a calculator on its website that gives you an indication of the costs of running on diesel, petrol, biodiesel, electric battery or fuel cells.
Rolling downhill or approaching a junction with the car out of gear used to be a common practice to save fuel. But the AA strongly advises against it. “You lose the ability to suddenly accelerate out of tricky situations [and] you lose engine braking … These days, coasting is still inadvisable and changes in vehicle fuel systems mean it won’t save you fuel, either.”