Lower well-to-wheel costs and environmental impacts are in Audi's sights with the announcement of an A3 powered by synthetic natural gas and capable of over 800 miles on a single tank.

That would theoretically enable the five-door hatchback to drive from London to Edinburgh and back without filling up.

The supremely economical combustion system, called the g-tron in echo of the A3 e-tron plug-in hybrid that was set alongside it on stage at the Geneva Motor Show, equates to an output of less than 30g/km of CO2.

The environmental and financial benefits of burning compressed natural gas (CNG) instead of petrol are well documented, the most obvious of which is that it costs little more than half what petrol does at the pump.

Audi has gone one step further, synthesising a derivative of CNG at a special Audi-owned factory. The company claims that all synthetic CNG it produces there is carbon neutral.

The wider Volkswagen group is aiming to reduce its fleet average emissions to 95g/km by 2020 and sees drastic low-emissions technology as the only way to do it. Among its other channels of powertrain development are enclosed hybrids, full-electric systems and cylinder deactivation for internal combustion engines.