Back in June the European Commission set out plans to force car makers to cut their emissions over the next five years - proposals that may end up capping average CO2 emissions at 130g/km by 2012. That’s average across all the cars that leave the factory gates. Of course the precise rules are yet to be set – and indeed reports suggest that the gravy-train-riding eurocrats may yet yield to lobbying pressure from the big German auto makers (who build a large percentage of the world’s CO2 spewing luxo-barges) by creating variable limits according to vehicle weight. This would, of course, largely defeat the object.

Whatever the emissions targets end up being, meeting them will probably prove more difficult for some makers than others. If you’re Fiat, for example, it’s probably going to be a lot easier to become a CO2 miser than if you’re, say, Rolls-Royce. According to figures in The Telegraph, Fiat’s fleet average is currently 146g/km. At the other end of the scale the Porsche fleet, no doubt bloated somewhat by all those lumbering, ugly-butt Cayennes, puffs out 297g/km on average. Cough.

This probably partly explains why lots of top-end brands are up for sale. Ford, for example, will find it a lot easier to meet a low average emissions level across its entire output once it no longer has to drag along the gas-gargling boat-anchors made by its Jaguar and Land Rover units. The similarly unthrifty Aston Martin has already been heaved overboard.

Talking of Rolls-Royce, current owner BMW (average output 190g/km) may have its work cut out to meet whatever emissions targets emerge. It currently builds a range that’s big on high-performance, luxury sports saloons and low on lightweight city runabouts. It’s slimmest model, the petrol Mini, weighs 1065kg. The Fiat Panda weighs 20 per cent less. And weight is to reduced CO2 what cheeseburgers are to reduced waistlines.

One way BMW hopes to limbo under the limit without having to resurrect its old bubble-car sideline is to nip away at the problem a little at a time. To this end its 1-series and Mini models are gaining a clever start-stop system developed by the spectacle-wearing boffins at Bosch.

Start-stop systems switch off the engine when it would otherwise be idling, halting the burning of fuel altogether when it is serving no purpose in getting from A to B.

Obviously the real trick is getting the engine to start reliably and quickly when you want to move off again – nobody wants to sit immobile at a green light in their shiny new BMW as the engine makes impotent revolving noises under the bonnet.

To this end Bosch supplies the full set of clever bits to make it all work: a special starter motor, a sensor to detect battery charge levels and ensure there’s enough oomph to get going again, and additional software for the black-box engine control unit.

And it makes a difference. According to Bosch, fuel savings can be as much as eight percent in city driving - a figure that arises from the ECE15 urban component of the New European Driving Cycle (NEDC) assessment. ECE15 simulates a typical cross-town journey of 7km interrupted by twelve 15-second stops. No doubt fuel savings will be better still in chronically clogged cities like London – where you’d be lucky to travel 700m without stopping 12 times.

The result is an impressive 118 g/km official score for the Mini Cooper D - well below any limit the EC might impose in 2012, and so helping to counter a few 7-series behemoths.

Halting an idling engine to save fuel is not a new trick, of course. Volkswagen’s 1985 Polo Formel E model was a pioneer, albeit one that faced a lot of consumer resistance. Maybe now, 22 years later, an overheating world is finally ready to hear the engine die every time the car comes to a halt.