Some cars are deceptively proportioned. I'm always astonished at how tiny a Lamborghini Gallardo is when you see one in the flesh. It's shorter than a Ford Focus and the styling seems artfully miniaturised. The Mercedes GL420 CDI hides its bulk well too. It looks a big car. When you step up to it, you realise that this diesel-powered 4x4 is a positive leviathan.

It's fully 508cm long, making it nearly 13cm trickier to slot into a parking bay than a full size Range Rover, the overall length being the same as an Audi Q7, a vehicle that also boasts seven seat capacity. Unlike the Audi, which is a mere 173cm tall, the GL stands 184cm off the ground. Park one outside your house and your neighbours may be on the council website checking out their Right to Light regulations.

Naturally something of these dimensions requires quite an engine to shift it and the GL420 CDI doesn't disappoint in this regard. Mercedes has boxed clever with this V8 engine, lining it up as a tempting alternative to the six-cylinder units offered by both BMW and Audi. Even Range Rover has been pipped to the market, their TDV8 engine's 272bhp looking a little asthmatic next to the brawny 302bhp Benz. With a massive 516lb/ft of torque at its disposal, the Merc's engine is top of the second tier behind the freakish Volkswagen V10 TDI and Audi V12 diesels.

It's also an engine which appears to render the GL500 virtually redundant. Let's start with economy. The GL500 driver will require a featherlight right boot to get anywhere near its quoted 21.1mpg. Drive 'normally' and you'll be looking at closer to 16mpg. With the GL420 CDI, you should get to within a couple of mpg of its quoted 25.1mpg figure without having to freewheel down hills. While this still won't make it the apple of the green brigade's collective eye, it's better than you'd expect from something like a Ford Focus ST. Emissions aren't eyewatering, with the big Merc returning 307g/km of carbon dioxide (GL500 331g/km).

"The Mercedes GL420 CDI is more responsible than its bulk carrier dimensions at first suggest"

It's properly quick too, the diesel engine and all-wheel drive system acting in concert to catapult the GL420 CDI off the line to notch 60mph in 7 seconds. A 143mph top speed will make interContinentalcruising easy while the 7G-Tronic seven-speed auto slurs beautifully between ratios. If you want the best automatic gearbox in the world, you're looking at it right here. Advantage Mercedes. Basically you're being offered the performance and fuel economy of a decent GTi hot hatch coupled with the sort of interior space that you'd need to put Old Trafford on wheels to better. The catch comes when you have to sign for one. At £63,075, you'll be hoping that the market for big 4x4s doesn't take a nose dive during your tenure of ownership.

Although it wears a German badge, make no mistake that the heart and soul of this car is pure stars and bars. Built in Tuscaloosa, Alabama, the GL is aimed at the well-heeled buyers of New York, California, Washington and Florida, customers who demand a vehicle with the front to face down a Cadillac Escalade or a Lincoln Navigator. Make no mistake, the GL is a serious piece of real estate.

Much of the GL420 CDI's appeal will hinge on practical aspects and Mercedes have worked hard to endow this 4x4 with family-friendly features. It's far more spacious inside than their R-Class sports tourer and features a centre-row bench seat with four-way backrest adjustment which is claimed to provide far better seating comfort for up to three passengers. The wider section of the 1/3:2/3 split-folding seat has a folding armrest with cup holder and compartment. The backrests and the cushions can both be folded down, for transporting bulky loads while the divided seat cushion folds forwards in individual sections, then the backrests are folded forwards to create a flat surface level with the load floor at the rear.

A newly developed EASY-ENTRY system provides convenient access to the optional third-row individual seats. To use this facility, the narrower backrest section of the bench seat is released and folded forward together with the cushion. If the front passenger seat is in a further back position, it automatically slides forward. When the third-row passengers have taken their seats, the bench seat can be folded back into position and the front passenger seat returns to its original place.

Equipped with the third-row individual seats, the GL-Class is a full-size seven-seater. Either one or both seats can be electrically folded to create a continuously level load floor which, when the centre seat row is folded down as well, extends right the way through to the front seat backs. Customers who do not opt for the seven-seater version get an additional stowage compartment under the load compartment floor. The optional rear-seat entertainment system, comprising two colour monitors with headphone jacks in the back of the front head restraints and a CD/DVD player under the centre bench seat, allows rear-seat passengers in the GL-Class to listen to or watch their own choice of music or films. To extend the entertainment programme even further, external audio/video sources or a games console can be connected to the system. It's a great way of keeping the kids quiet on a long journey.

Size isn't everything but the GL420 CDI backs up the brawn with formidable brainpower. It's definitely a little cleverer than the eco warriors who sneer at it, not realising that it sups less fossil fuel than a 2.5-litre Mondeo. From up here, the social stigma looks a long way away.