We have been cruising at exactly 120 kilometers per hour (75 mph) when we finally pass under an electronic overhead sign that displays the readout we've been waiting for: a circle with slashes through it. At virtually the same moment, the speed-limit readout on the head-up display also gives the go-ahead, and we floor it. The car gathers speed quickly but with little drama, and we settle in at a comfortable 240 km/h (149 mph), although there is more available. This seems like enough given the presence of slower traffic, and, indeed, every time someone pulls into the far-left lane to pass, we give the big carbon-ceramic discs a workout but then quickly regain our pace. Driving on an unrestricted-speed autobahn is an intense experience for the driver, but the two passengers seem unfazed. This is exactly the situation for which the BMW M6 Gran Coupe was born: high-performance driving while carrying two (or three) passengers in relative comfort.