What's the most efficient highway travelling speed?

Driving slower will save you some fuel, and a little money. But are you prepared to cop the time penalty?

Potential Fuel Saving: Up to nine per cent

Here's a quote from the Federal Government's Green Vehicle Guide website: “High speeds result in high fuel consumption. At 110km/h your car uses up to 25 per cent more fuel than it would cruising at 90km/h.”

Okay, we got only nine per cent variation in the Calais when we tried it, but that’s hardly the point. Driving at 90km/h in an environment where a great many motorists (including B-doubles) are ripping towards you from behind at a closing speed of 20km/h – at least – is terrifying. Some of those people are not concentrating. You become a de facto target. It’s like dicing with death. On Valium.

We did this for a total of 120km for this test – for the first and only time. Never again. It is impossible to reconcile the Green Vehicle Guide’s ‘advice’ with other pertinent real-world considerations, like personal safety. Driving well below the speed of the prevailing traffic flow is dangerous, as numerous overseas studies have proposed. The average Australian spends about $2400 on 1500 litres of fuel annually, and around half of that is driven outside the capital cities. Nine per cent of half of $2400 is $108 – call it $0.30 per day – hardly worth risking your neck for.

And since fatigue is the number-two killer, allegedly, on our highways, how is it possible to reconcile the potential saving (30 cents a day) with a system that would increase the travel time on our highways by 22 per cent? Clearly the costs here, potentially, outweigh the benefits by a great deal.