The fastest accelerating B-class is the electric B 250e, its instantaneous torque pushing the car to 62mph from rest in a respectable 7.9 seconds. It's largely irrelevant to British buyers that it tops out at a modest 100mph, but the way it gets up to that speed is deeply satisfying.
The conventional models in the line-up are rather disappointing in comparison - and not just because of the raw figures. The quickest of the 'normal' cars is the B 220d, posting an 8.3-second 0-62mph on the way to 139mph. The 4Matic version accelerates just as quickly despite its 70kg weight penalty, thanks to extra traction off the line. It feels fast enough for the type of car it is, but the diesel engines are all a bit gruff and they certainly don't encourage the driver to trouble the red paint on the rev counter.
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Wear ear defenders and it's theoretically possible to get the B 180 CDI to 62mph in 11.6 seconds (11.9 with the auto), while the B 200d does it in 9.9 seconds (9.8 with the auto). The turbocharged 1.6-litre petrol engines aren't much faster, posting a best time of 8.4 seconds in the automatic B 200 or a worst of 9.3 seconds for the manual B 180. They do fare better at the top end and at least they're smooth and quieter than the diesels.