Driving efficiency is a topic that often gets overlooked, but it's a crucial aspect of saving money on fuel. According to the federal government's National Fuel Security Plan website, there are several ways to improve your driving technique and tinker with your car to save on fuel. These include shedding weight, going slow, and being gentle on the pedals. But what savings can be made, realistically, short of buying a new car? Anywhere between 10 per cent and 30 per cent, says a senior lecturer in mechanical engineering. Last month, the Department of Infrastructure and Regional Development launched a fuel-saving tips website as part of its $20 million advertising blitz. Federation University's Truong Phung examined the National Fuel Security Plan advice and concluded that, while multiple variables impacted the effectiveness of various fuel-saving strategies, the principles were essentially sound. "It is good advice. I think they must have consulted with engineers and researchers," Dr Phung said. "The range of savings would be huge." There is a thing called hyper mileage. If you remove everything you don't need from a car to make things as light as possible, and you try to accelerate the car as slowly as you possibly can, you can make a significant impact on consumption. There is an American experiment you can find online that showed you can save more than 15 per cent. That's pretty decent. You may annoy other motorists, but you will save money. The department's fuel-saving tips page offered a range of actions to improve any vehicle's fuel efficiency, along with driver behaviour advice to also reduce fuel usage. Dr Phung said gains from each piece of advice would potentially be cumulative. Tyre pressure The Department of Infrastructure advice to inflate tyres to the highest pressure recommended by the vehicle manufacturer could improve fuel efficiency by up to 5 per cent. But Dr Phung said there were trade-offs with higher tyre pressures. "It's conflicting. You want traction, which means friction, but you also want to minimise rolling resistance," he explained. "With higher pressures the tyre will be lifted from the road and the contact patch [with the road] will be smaller, which means less resistance and tyre wear. With the lower pressures, however, the tyre will be squashed and wrapped around the road surface, and you may have more traction. It also acts as extra suspension, so on a rough road it will be more comfortable." Weight-loss gains Increased weight forces a vehicle's engine to work harder, so reducing weight will improve fuel efficiency. There is a compromise between savings and convenience, though, according to Dr Phung. Removing a car's spare wheel, jack and other equipment would reduce mass but potentially leave you stranded in an emergency. "Many new cars now have a tyre repair kit or run flat tyres so you don't carry a spare," he said. "That is one way car makers improve fuel economy." Aerodynamics and drag The government website suggests removing items such as roof racks and boot lid spoilers to reduce drag, which improves fuel efficiency by up to 17 per cent. Dr Phung said, while this was likely true, it was also not practical in many situations. "If you carry pushbikes on a regular basis, for example, to remove roof racks every single time would be very time-consuming, unless you have a clip-on rack," he said. "Even more so with things like bull bars." Aerodynamic efficiency was one factor that went into car design but hardly the only factor. "It's a complex situation," Dr Phung said. "The main thing for designers is probably the appearance of the car. That comes first so people can see and recognise the brand. After that, they will try to minimise the frontal area of the car, to reduce the skin friction and the drag coefficient." A sports car or sedan has a lower drag coefficient than a truck or SUV, which may have a front area 1.5 times higher being pushed by the same engine. And drag is multiplied the faster you go. If you double your speed, the drag force may be quadrupled. Air-conditioning Adjusting a car's internal temperature can affect fuel economy because air conditioning draws on engine power, according to the online advice. Again, the situation is not always simple, though. Dr Phung said opening a window to cool the car rather than use air conditioning was false economy at higher speeds because an open window created more drag than a closed one. Driving style The government suggests reductions in speed and acceleration could reduce fuel consumption by between 10 and 40 per cent in city driving and between 15 and 30 per cent on a highway. Less acceleration and less braking means less wastage, with a car's engine usually designed to operate at peak efficiency between 1,500 and 2,500 revolutions per minute. Dr Phung backed a sweet spot of about 90 kilometres per hour for maximum efficiency, but that was heavily dependent on other factors such as aerodynamics. "It comes back to the amount of drag force," he explained. "The government website compares 90km/h with 110km/h. It's a complex formula but, if you increase your speed by 20km/h, the drag force is increased by about 30 times or more because it compounds." Efficiency gains from driving smoothly are even greater with petrol-electric hybrid cars, which did not use any fuel at all below a certain speed (it varies between brands and models) when the electric motor and battery are doing all the work. Dr Phung said using higher octane "premium" fuel was necessary in some car models but would not generally improve fuel economy in cars designed to run on regular 91 octane petrol. "You have high-performance cars where you have to use 95 or 98 octane petrol, it's because the compression ratio is different," he said. "It means that the piston must travel further to compress and you don't want the petrol to ignite when it is compressing because that causes engine knocking and can damage the engine. With a car that is designed for 91 octane petrol the compression ratio is lower, and it will run fine on 91. There's no point in spending extra money for a thing you don't use," Dr Phung said.
