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Improving Fuel Economy

Achievable real world stuff

by Julian Edgar

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This article was first published in 2008.

In these days of high fuel prices and increased concern about greenhouse gas emissions, lots of people want to know how the weekly fuel bill can be reduced. Let’s take a look.

Driving Style

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I bet more than half of you saw the sub-heading ‘driving style’ and wanted to skip straight past it. I am glad you didn’t. Because of all the topics covered in this article, improving fuel economy by altering driving style is the single easiest, cheapest and most effective technique.

I once had a job photographing people in restaurants. The pics would be taken then sold to the people as souvenirs of their night out. I soon found that the job was a lot more about ‘sales’ than ‘photography’, so after a short time I gave it away. But the relevant part for this story is that my boss, a woman in her mid-30s, often drove me from restaurant to restaurant.

And her driving was a revelation.

She looked ahead literally only 20 or 30 metres; every red light involved heavy braking – braking that was so strong that it would have caused a skid had the road been wet. (I never rode with her when it was raining – just as well.) When the traffic light went green, she just stomped on the throttle and away we went.

The fuel consumption of that sort of driving is just horrendous.

Even if you don’t drive as badly as she did, many people could improve their fuel consumption (and decrease the consumption of tyres and brakes) by driving better.

When a car is being driven for good fuel consumption, the acceleration and braking are so smooth that a full glass of water on the dash shouldn’t spill a drop. Think about that for a moment: it means that when slowing for a red traffic light, the throttle lift-off should be sufficiently far ahead of the light that the transition from throttle-on to throttle-off to braking is one smooth continuum of deceleration. In fact, in normal driving, if you need to swap your foot straight from the throttle to the brake pedal, you’ve made a driving mistake.

When the light turns green, you should (in a manual trans car) use plenty of throttle but change up early. It depends on the car, but that might mean making your shifts at 2000 or even 1500 rpm. If you are using a small throttle opening and changing-up at high revs, you’re wasting fuel. In an automatic trans car, use whatever throttle behaviour keeps engine revs as low as possible.

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On the open road, acceleration and braking are not the main driving style factors affecting fuel economy. Instead, the slower you go, the better will be the fuel economy.

Now obviously common sense comes into play here – no one is going to like you much if you drive down a 110 km/h speed limit dual lane highway at 80. But as a specific example, a trip I commonly make is on a 4-lane each-way, 110 km/h freeway. If I am not in any hurry, I’ll sit in the slow lane at 95 – 100 km/h.

Summary:

In urban driving you should drive smoothly and look well ahead, predicting traffic behaviour rather than reacting to it. In country road driving, travelling slower is the easiest way of improving fuel economy.

Car Preparation

I know a bloke of the old school: he carries around in his car’s boot a heavy tool box stuffed full of weighty bits. It’s probably been in the boots of all his cars for the last 30 or 40 years: back then, it was perhaps a necessity when travelling a long way in vehicles that were not as reliable as today’s cars.

But such an unnecessary load simply wastes fuel. Each time he accelerates, he’s also bringing up to speed a tool box that would be better in his shed.

The same can be said of any extra load in your car. If you drive primarily in urban areas, consider removing the spare wheel. I reckon I’ve used my spare once in the last ten years; that’s been ten years of carrying the thing around for very little use. (However, in small cars be aware that the spare wheel may be part of the car’s crash safety strategy in a rear-end collision.)

Heavy subwoofers are another weighty addition that perhaps you can do without.

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At highway speeds, aerodynamic drag uses the majority of power being developed by the engine. Modern cars are vastly slipperier than cars of yore; however, those advantages can be lost if the car wears roof racks, is driven with the windows open, or someone has removed the engine bay undertray eg to make oil changes easier.

Talking about oil changes, the thinnest oil recommended by the manufacturer will reduce internal frictional drag. When it comes time to replace the current tyres, consider fitting low rolling resistance tyres. Tyre pressures should always be at the maximum end of the manufacturer’s recommendations – however, I do not recommend some of the extremely high pressures that people chasing fuel economy sometimes run. (I’ve seen suggestion of pressures of 50 psi – the ride will be terrible, the handling suspect, and I wonder if the insurance company would even cover you if you had an accident.)

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A fuel-efficient car should be well maintained: the days of 10,000km “tune-ups” are long gone but by the same token, replacement items like spark plugs, spark plug leads (if fitted), fuel filters and air filters should be replaced at manufacturer-specified intervals. The EGR valve and injectors should also be kept clean. We do not recommend fitting “upgraded” aftermarket filters or sparkplugs; if the manufacturer’s original equipment parts are too expensive, buy conventional replacement parts from a well respected non-OE supplier.

Summary:

Keep the car as light as practicable and safe. Make sure that its aerodynamic form is not degraded and keep the tyre pressures at the upper limit of the manufacturer’s recommendation. Keep the car’s engine well maintained.

Car Choice

‘Car choice’ – why is this being placed last? Shouldn’t it be first? The reason we’ve put this at the end of the story is that is usually what happens in real life – people try to improve the fuel economy of the car they have, rather than having selected the car on the basis of fuel economy.

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Some things don’t change – and one of those is this: A smaller engine car will give better fuel economy than a car with a larger engine. Of course there are exceptions, but the general rule is very broadly applicable. If you want lots of performance, pick a car that has a turbocharger on that small engine. For reasons we’ve covered elsewhere (see Turbo'd for Fuel Economy), current turbo engines tick all the right boxes – driveability, performance and fuel economy.

Reflect carefully on the criteria you want a car to match. If you’re a performance enthusiast, think past the concept of ‘performance’ meaning only acceleration. Performance in terms of handling, in brakes, in steering feel – none requires a thirsty engine.

If you take an annual holiday that goes off-road, how do the figures calculate if rather than buying a 4WD, you hire such a car for a week or two each year? (And keep in mind that you’ll also get a current model each time!)

How many seats do you really need? What load space versatility to you require? If you want a tow-bar, what is the maximum load you frequently tow?

As an automotive journalist, I drive a lot of different cars over the course of a year. But you probably don’t. Is it worth spending $500 or $1000 to hire a range of different cars and then experience them in your own driving environment, with your own requirements put into effect? It probably is – and you’ll learn far more than on a dealer’s drive-around-the-block with the salesperson twittering in your ear. Even if you’re after a second-hand car that cannot be hired, sampling a selection of current model hire cars of different types will at least open your eyes to what actually works and doesn’t work for you, your family and your driving circumstances.

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Before we leave this topic, many people simply don’t know what different cars are like. Some think that current turbo diesels are slow and unresponsive – typically they’re not. Some think that hybrid cars are incredibly gutless – usually they are not. Others think that large cars always have lots of interior space – but many 4WD wagons have very poor space utilisation. Make sure that you’ve experienced enough different types of cars to make a meaningful car buying decision.

Summary:

Think outside of your preconceptions. Experience what cars actually exist and what they do well and poorly. ‘Performance’ is a lot more than just acceleration: with the exception of small engine turbocharged cars, ‘acceleration performance’ invariably means higher fuel consumption.

Conclusion

We don’t think any of the points made in this article are groundbreaking. Bluntly, they are not. But if you select the car carefully on the basis of the real needs it must satisfy, keep it well maintained and drive it well, you will be getting the best fuel economy possible in your driving circumstances.

And that’s a pretty good thing...

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