We’re heading into the season when many people think about changing their car, for whom there are a few fuel-saving angles worth considering, even if you’re not a confirmed fuel economy fan. Tempting finance deals mean many buyers will choose a brand new car, in face of the better value of a nearly new, or relatively young used car. Bear in mind that you won’t get the best fuel economy out of any brand new diesel engine until it has covered quite a few miles, as much as 30 to 40,000 miles, which might be the very mileage of the old car that you’re thinking of trading-in!

So, looking at issues directly affecting fuel economy, the class and size of car ñ big or small, hatchback, estate, or SUV, what engine size, and manual or automatic transmission, are big factors. Other issues, like maybe whether 2WD or 4WD, can be similarly significant. The key factor is possibly going to be the size and class of car, and it may have a bigger effect than you are expecting. Compared with a medium hatchback, just about any medium sized SUV is going to be taller and heavier, meaning it will burn more fuel, however you drive it. It may even need a bigger engine for good performance. Choosing a Ford C-MAX instead of a Focus hatchback or estate, or a SEAT Ateca in place of a Leon hatchback or ST estate, you’ll use 10 to 15 per cent more fuel, say owners reporting their figures to ìHonest John’s Real MPG.î (www.honestjohn.co.uk/realmpg)

That then raises the issue of engine size and power; few cars are underpowered these days, and many arguably overpowered. So, if you’re choosing between a 74bhp or 94bhp 1.5 TDCi Ford B-MAX, or between a 108bhp 1.6 TDI or 148bhp 2.0 TDI Golf, does it make a big difference to fuel economy? It depends. If you’re more of a press-on driver, you’ll probably use more fuel with a bigger engine, and enjoy it. But figures show there’s little difference between alternative engines when they are driven similarly. In some conditions, though, like on the open road, a heavily laden car may be more economical with a more powerful engine pulling higher gears, whilst in town a smaller engine may save quite a bit of fuel.

Manual or automatic? It’s a question being asked more and more these days, with newer twin-clutch automatic transmissions available, along with more economical traditional automatics. Scan Diesel Car’s data pages for the ìofficialî mpg figures, or look at real fuel economy figures and you’ll see that many automatics almost match their manual counterparts, and some even better them. Real life owner figures say the penalty for going automatic is around five per cent, or 3 to 5mpg, whilst motorway automatic drivers may even save money, with the latest seven, eight, or nine-speed gearboxes that cruise economically in very high top gears. A BMW 318d eight-speed automatic is 2.9mpg less thirsty than a six-speed manual, but do check the figures out, as there are notable exceptions.

Four wheel drive/all-wheel drive in place of 2WD? On paper (the ìofficialî figures) a 4WD will be more costly in fuel. An AWD Jaguar XE 2.0d takes a 7mpg hit, for example, while an Audi A4 2.0 TDI quattro around 6mpg, and a BMW 320d xDrive around 4.5mpg over 2WD. But the real-life figures seem to be closer ñ a penalty of nearer to 4 to 5mpg ñ so choosing AWD won’t cost you much more, if you suffer severe winter weather regularly, and you won’t waste fuel spinning your wheels in the mud and snow!

Finally, let’s consider the eco model variants that often sell at a price premium. Their official figures claim big fuel savings, but owner figures for two popular models we checked show the extra costs don’t offer much of a payback. Their gains usually come from their low rolling resistance tyres, and from improved aerodynamics that won’t offer significant benefits at lower speeds. The special tyres alone can give you another 3 to 4mpg though, and you can fit them as replacements on any car; but you won’t find them available in your size if you’ve chosen optional big alloy wheels with low profile tyres! Apart from their higher replacement costs, the extra width and aerodynamic drag of these big tyres means that they actually increase fuel consumption. It explains why BMW fit unusually narrow 155/70/19-inch low rolling resistance tyres to the electric i3 models, to minimise the rolling resistance and drag, and help get more range out of the battery.

Victor Harman