What uses more petrol distance or time

Advice

A simple guide to finding the best speed to maintain the best fuel economy

16 May 2019

Does driving slower actually save you fuel? Well, that entirely depends on what speed you're meant to be doing in the first place.

Driving on a highway? Going slower technically saves fuel because at high speed your engine needs to work harder to overcome drag from wind resistance, your tyres and transmission, and that drag increases exponentially the faster you go.

Reduce your speed, then less power (and thus fuel) needs to be expended to overcome that drag. Simple, right?

However, travelling at a slower speed also means your journey takes more time, so your engine is burning fuel for longer than it otherwise would. Common sense informs most people that crawling along at 20km/h is not an efficient way to drive a car.

So a big part of driving efficiently comes down to identifying your car’s most efficient speed, which is the sweet spot between where both travel time and drag each have the least impact on fuel economy – as demonstrated in the graph below showing an optimal speed around the 80km/h mark.

This is not a one-size-fits-all exercise though, with different cars producing more or less drag (technically known as their drag coefficient) depending on their aerodynamic profile.

For example a boxy SUV (which has a higher drag coefficient) will be less efficient at higher speeds than a sleek sedan with the same weight and power, meaning its peak optimal consumption speed will be a little lower. Even the kind of tyres can make a big difference to a car's most efficient speed.

You’ll need a white coat and slide rule to work out your car’s exact optimal speed, but fortunately this isn’t necessary.

As a rule it’s going to be in top gear but at low revs, below 1500 rpm, or just above the point where you’d need to downshift.

If you have an automatic transmission check the lowest speed it will constantly sit on in the highest gear before it shifts down – for best results do this in eco mode if your car has one.

In most cars the sweet spot is about 80km/h with about 8km/h leeway either side of that to maintain reasonably good efficiency.

Cars with bigger engines like V8s will usually be happiest sitting between 90 and 100 km/h, and especially-aerodynamic vehicles like hybrids and EVs will be in the same boat. Sports cars can be a mixed bag - they're generally shaped to minimise drag, but some design features also act to increase downforce - which induces drag at high speed.

Things to remember

• If your car has a real-time economy gauge, use it on a relatively flat road to get an idea where your optimal speed is.
• Use cruise control to help maintain a constant speed.
• Of course travelling at a constant isn’t always possible and your car will always need to rev harder to negotiate hills.
• And there will be times when you’ll need to slow down, and speed up again, which also hampers efficiency. If you need to slow down try avoid using the brakes by taking your foot off the accelerator to coast and get back up to speed slowly rather than stop and start with a burst. Keep your eyes up and try to anticipate traffic movements well in advance.
• Your cars fuel efficiency is also affected by weight and tyre rolling resistance. You want less of both of those things.
• If you want to deliberately drive frugally remember to consider other drivers if it means you'll be going slower than the speed limit. Don’t hold up other drivers and stay out of the right lane. Not only will you frustrate other road users, you might end up with a fine.

COMMENTS

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