For generations, weight has been one of the greatest enemies of car designers, usually forced on them by the demands of their colleagues in sales and marketing, especially when it comes to fitting out something in the luxury class.

Bigger windows (glass being heavier than sheet steel), electric seat adjustment, more comprehensive corrosion proofing, air-conditioning, wider wheels and tyres - all these things unavoidably add on the avoirdupois.

A lighter weight material is there to be used. Aluminium has been specified for individual components since the late 19th century, but it took until 1989 for one of the Japanese companies to market a limited-production sports coupe with an all-aluminium body.

The Audi A8 remains the only volume production saloon which has not only an aluminium body but also a full aluminium frame.

Middle cars in the six-model range are the A8 3.7 at #43,350 in standard trim and #46,793 as the Sport. Since the engines continue the overall theme by being built of aluminium too, these are the lightest V8 saloons of their size on the market.

The original Audi V8 was never the success as Audi's first modern effort in the luxury class which the company hoped. Refined in many ways, the A8 is a better effort all round.

Styling is discreet, disguising the car's imposing dimensions until you see it close up. Cabin and luggage space, the leather and woodwork trim, and the layout of fascia and controls are all fine, although the fold-down front central armrests seem uncomfortably high.

In short-stroke 3.7-litre form, the engine which also appears enlarged in the 4.2 quattro peaks at 230bhp. That gives the A8 a definite power to weight ratio advantage over its direct rivals, and the engine is also particularly strong on mid-range torque.

Transmission is a five-speed Tiptronic system, which does take an edge off the performance compared with a manual. However, the A8 sprints to 60mph in just over nine seconds, has a test track maximum close to 145mph, and matches that with fuel economy as good as 39mpg at 56mph.

Left to its own devices, the transmission is one of the finest automatics on the market, responding well and making almost imperceptible changes. While not admiring the aluminium selector casing on the central console, I enjoyed the manual aspect of the Tiptronic system which some critics disparage.

Main road cruising is relaxing in an A8, and mechanical noise is subdued, although the test car creaked occasionally, somewhere at the left-hand side of the fascia.

Undulating country roads tackled at speed do not see the standard car at its most composed.

The extra-cost Sport version caters for harder drivers, although whether its stiffer suspension, wider tyres and naturally firmer ride go with what is essentially a well equipped, solidly built and dignified saloon is a matter of personal taste.