Both are versions of...

Both are versions of the forthcoming R-Class, but are actually different lengths. The bottom one is the standard car and has been seen before, but the top car is the all-new stretched wheelbase version. It will make its debut at next week"s Detroit Motor Show.

With an extra 23cm added between the wheels, the lengthened car will be Mercedes" most spacious model for rear seat passengers, and will rival the S-Class limo for comfort and luxury. The extra centimetres have been added to the rear door, which is why they are so difficult to spot.

When the R becomes a production reality, both long and short wheelbase variants will be sold across Europe, with the first deliveries expected towards the end of this year. Sharing its platform with the new M-Class SUV means it will also have four-wheel drive. Inside, both R-Classes are fitted with what the German designers are calling a 4+2 cabin, with four large chairs accompanied by two occasional seats at the rear. Those who don"t need such a vast interior will have the option of the smaller B-Class, which was first displayed as the Vision B concept at the Paris Motor Show last September.

Under the bonnet, the new models will be offered with a range of Mercedes" most powerful petrol and diesel engines. The top-seller is likely to feature the firm"s new V6 diesel unit, which will also be shown for the first time in Detroit. Developing 221bhp and 510Nm, the range-topping oil-burner will offer the optimum balance between performance and economy.