BMW has announced details of the third M3 derivative, ahead of its April launch. It'll be the first M car to feature a retractable hardtop, and also the first to carry the option of BMW’s new seven-speed transmission

Gearchanges are predicted with the optional M DCT gearbox. Two oil-cooled wet clutches are deployed, one controlling gears 1, 3, 5 and 7 and the other gears 2, 4 and 6. Gearchanges are made in a few milliseconds and the next gear selected in readiness, contributing to better acceleration, economy and emissions than that of the conventional manual car.

To facilitate town driving, the new M DCT gearbox uses a gradient detection system and Low Speed Assistant. These combine to ensure that the car won’t roll back on hill-starts and that the engine’s high output levels don't intrude into low-speed driving.

Power output is 420hp at 8,300rpm, with 400Nm of torque at 3,900rpm from its 3,999cc V8-power unit. All of which propels the car from zero to 62mph in 5.3 seconds and on to an electronically-limited top speed of 155mph. The combined economy cycle, not that we suspect buyers would be that interested, is a creditable 21.9mpg.

The new BMW M3 Convertible goes on sale in April 2008 and costs £54,655 OTR. Add £2,590 for the optional M DCT gearbox.