I like to think of myself as a green driver. In my quest to beat my car manufacturer’s official fuel consumption figures (so-called hypermiling), I smugly cruise at an energy-saving, diesel-powered 72kph. But am I really driving as efficiently as I think?
I drove — slowly — to Bedfordshire near London to find out. I had seen the intriguingly named Millbrook Proving Ground on the BBC motoring show Top Gear, when presenter Jeremy Clarkson et al thrashed some trucks around its 72km of twisty track. For the BP Ultimate green driving test, however, I was required to proceed super-sedately in a two-litre petrol Ford S-Max.
The greenest drivers complete the 12,6km course — a mix of stop-start city driving, hill starts, mountain tracks and motorways — on one litre of petrol. I would see how far a litre would take me. (Disappointingly, I would not actually come to a halt; the engineer’s laptop just beeped when I used up my litre, while two data loggers kept track of the vehicle’s speed and fuel consumption.)
Anthony Sale, a Millbrook engineer, sat in the back, quietly making notes about my driving on his laptop as I tackled the “city” stretch first. After coasting up to a few roundabouts in neutral — believing this would help my fuel economy — I came to the incredibly steep truck slopes. It took an enormous high-revving, fuel-gobbling wheelspin to heave the S-Max up the hill.
Next up was the twisting “Alpine” road. I trundled sensibly through the gears and not even 8km into the course, Sale’s laptop gave a disapproving beep. I had, in effect, run out of fuel. I still had one stretch left: the high-speed bowl. Here, surely, I could excel. I smoothly took the car up to 72kph and held it there in fifth gear.
Back at the test centre, I was ushered into the results room. Sale holds the record for the “perfect run”; he achieved 12km per litre.
My score, however, was a shock. If this test could be failed, I was a failure. I averaged just 8,6km per litre — 40% less efficient than the ideal score — and emitted 267g of CO2. Over a year (and 16 000km), my inefficient driving would cost me an extra £558,94 in fuel.
Despite trying to drive smoothly, I had braked and accelerated far more than necessary. The key to fuel efficient driving, Sale explains, is to keep your speed consistent. That requires concentration and anticipation. “It’s looking much further ahead, it’s lifting off the accelerator much earlier rather than keeping at the same speed and braking at the last moment.
“Imagine you’ve got an egg on the throttle and you don’t want to break it,” says Sale. Gear changes should have been smooth and progressive.
My supposedly fuel-saving habit of slipping the car into neutral to coast up to traffic lights was not only unsafe, but actually used more fuel than driving in gear. “What you really want to be doing is coasting in gear,” explains Sale, so that the engine is being turned over by the movement of the wheels. Put it in neutral and the engine has to use fuel to keep it idling.
So, green driving doesn’t slow you down. Despite driving 26kph faster than the optimum speed — 65kph in fifth gear — along the motorway section, I still finished the course more than a minute slower than the most economical drive because of my stop-start style. “Everyone thinks that driving economically is driving slowly,” says Sale. “It isn’t. It’s driving smoothly.
How to get the most from your fuel
Shut your windows and turn off the air con. You can leave windows open at speeds below 65kph, when there is a minimal increase in fuel-sapping aerodynamic drag. Air con increases fuel consumption.
Check your tyre pressure regularly. Under-inflated tyres increase the rolling resistance between the tyres and the road, causing you to use more fuel.
Remove car clutter. Removing clutter and roof rails when not in use can reduce fuel consumption.
Look further ahead. The key to fuel-efficiency is driving smoothly. Better anticipation of hazards ahead means less acceleration and less braking.
Coasting to a halt in gear is a big fuel saver.
Maintain your car. Change your air filter annually and ensure your engine is working effectively.
Stop driving. Short drives are uneconomical: when the engine is cold, it uses more fuel than when it reaches the optimum working temperature. Catalytic converters, which reduce exhaust emissions, are also less efficient when cold. —