Audi's A5 now comes with stop-start technology on the 2.0 TDI and TFSI engines. Good news for trees and marmots, because it means CO2 emissions drop by 5g/km and fuel economy is improved by three per cent. In cold, hard data, then, the A5 2.0 TDI will do 53.3mpg and produces 139g/km. Not world-beating, but it's a start...
Or maybe not. Because BMW has just launched the 320d EfficientDynamics saloon. The figures for this read so much better - 68.9mpg and 109g/km, with only 6bhp less. Poor old Audi. Just as it finally comes out with some stop-start technology of its own, BMW moves the game a full three steps ahead in one fell swoop.
Not that Audi's system is actually bad. Noise insulation is much improved recently, so the diesel rattle is well isolated from you in the cabin. And the stop-start works beautifully. Like most of these systems, it's only available on the manuals, so you pop the A5 into neutral and take your foot off the clutch to turn off the engine. In reverse-order, by the time you've depressed the clutch the diesel will be up and running again. Crucially, it also keeps the aircon going, provided you haven't got the temperature set too low.
Ultimately, though, it doesn't matter. BMW makes a far greener car and Audi is still at least a step behind.