The more observant readers will spot that we have deliberately not chosen an 'our pick' for this test because the Mercedes GL420 CDI has no specific rivals.

Mercedes can truly claim a first with its prestige seven-seat 4x4 powered by a V8 diesel.

Others prestige brands have a seven-seat 4x4s, but none with such a powerful engine, and others (VW) have powerful prestige 4x4s, but with only five-seats. Only next year will the GL420 CDI gain rivals; namely the Audi Q7 and (probably) the next BMW X5. Land Rover is also expected to bring a V8 diesel Discovery and Range Rover to the market.

So in September Mercedes will launch the GL, an off-roader that's more than 300mm longer than the M-class, a foot in old money, making it more than five metres long.

The reason to pick the new V8 diesel over any other engine from the GL line-up is logical. It provides the best combination of performance, economy and distance between fills.

The GL420 CDI will give you 24.4mpg, 0-62mph in 7.6 seconds and a distance between fuel stops of 550 miles. There is also a less powerful and almost as refined GL320 CDI which does much better on economy and can go a further 100 miles between fills, but feels a touch sluggish for a prestige car. The 388PS 5.0-litre V8 petrol is on paper quicker with a 0-62mph time of 6.6 seconds, but doesn't feel much faster than the V8 diesel and has the penalty of getting 450 miles per tank.

Interestingly, given the lack of serious off-road use prestige 4x4s get in their lives, Mercedes has opted to fit the GL with its Off-Road Pro pack that's an option on the smaller ML, as standard. That's because Merc is pitching the car as an all conquering vehicle to take on seriously competent cars such as the Range Rover. Off-Road Pro allows the driver to vary the height of the vehicle and lock various diffs to help get out of or into tricky situations.

One trick no rival has, is standard powered folding rear seats which are the easiest of any 4x4 and incredibly fast too.

While the inside of the GL is mainly of the highest quality, there's little wow factor such as you'd find in the design of a Range Rover. And whether you're driving the GL or just a passenger, the lasting impression is its huge size. The off-roader drives well with little body roll and total refinement but there's just no getting away from it, the massive GL makes the M-class feel like family hatchback.

At £63,000 the GL is an expensive car but we won't find out how expensive it is until it gains some true rivals next year.

Subscribe