It’s interesting to look at published fuel consumption figures for some vastly different cars and discover how sometimes they seemingly defy logic.

Take Volkswagen’s Passat BlueMotion 1.6 TDI saloon and the Ford Ka 1.3 TDCi, both with EC Combined economy figures of 68.9mpg; the identical figures are quite an eye-opener, considering that the Passat weighs 450kg more, and is nearly four feet longer than the Ka, and you might argue that the figures make a nonsense of buying a small diesel like the Ka for better fuel economy.

But the Ka is far cheaper, nippier, and a more wieldy proposition for serious one-up urban motoring, and it’s more a case of praise for the Passat, which matches the boastfully “green” BMW 320d Efficient Dynamics in both the official fuel figures and the average “real life” figures of around 59 to 60mpg variously reported by its owners.

Of course it’s far easier to get good economy on long open road runs, so it’s unfair to judge the Ka harshly for the unimpressive 50mpg that its owners report, as their motoring is probably mostly urban, while the Passat is going to be largely used on the open road.

It all makes sense in that Passat and 320d ED owners are typically returning around 75 per cent of the official Extra Urban figures, while many city-bound Ka owners achieve as much as 85 per cent of the Ka’s official urban figure, which is surely quite commendable?

Thinking further about such contrasts, economical open road driving requires the avoidance of energy-wasting acceleration and braking, and aims at conserving momentum whenever possible; so a car’s weight is rather less important and its aerodynamics are more important, particularly when average road speeds rise.

In urban motoring, the position is reversed, because the mass of the vehicle is being constantly accelerated and braked, using lots of fuel in lower gears that are not too economical, while the low speeds involved mean that aerodynamics have very little influence.

It’s little surprise then that it’s not that easy to design a car that’s really economical in all types of motoring. For urban fuel economy, you really need something light and therefore relatively small – maybe like a smart, or a Fiesta; or maybe a hybrid Toyota Prius that recovers decent amounts of waste energy – but then we know how thirsty even an aerodynamic Prius can be if you start cruising the motorways at 70mph plus!

For open road fuel economy, with comfortable seating for four or five, and decent luggage capacity, then aerodynamic diesels like the Passat or BMW just can’t be beaten.

But what determines how aerodynamic a car may be is not just how slippery the shape is (its Cd) but also the frontal area that it presents to the resisting air. The substantial amount of fuel needed to overcome that resistance explains why big SUVs are inevitably thirsty in fast motoring – even when, like the new Range Rover, they have been on a serious weight-loss diet.

Although 450kg lighter, engine for engine, the more slippery new model Range Rover SDV8 is only 2.4mpg more economical than the old TDV8. But the Range Rover’s amazing weight loss has made it possible to offer a 650kg lighter 255bhp 3.0 TDV6 model with near enough the same 0-60mph and 130mph top speed as the old 308bhp TDV8.

The EC Combined figure soars from 30.1mpg to 37.7mpg, the biggest gains coming from an Urban figure that leaps from 24.6mpg to 33.2mpg, which highlights what we’ve said above about the importance of vehicle weight in urban motoring.

Such beneficial trends for reducing vehicle weights, using lighter high-strength steels and aluminium to replace steel, along with smaller and less thirsty engines are set to be a key feature of many forthcoming new models.

But weight reduction in isolation makes little difference to open road fuel economy, and the economy tip of emptying your boot of tools, toys, and other clutter, or leaving poor old Granny at home on the Sunday afternoon run, has very little effect in the grander scale of things!

Victor Harman