THE Chevrolet Volt, a compact, petrol-electric hybrid launched by GM a year ago, was already selling poorly before it emerged last month that its batteries had caught fire in crash tests. GM is likely to fall several thousand short of its target of selling 10,000 Volts this year. Despite subsidies, electric cars and hybrids (which can run off batteries or a generator powered by an engine) are shifting sluggishly.

The Volt's battery problem should be fairly easy to fix. But the profusion of hybrid and all-electric cars now hitting the roads faces a far bigger challenge. Petrol- and diesel-engined vehicles are becoming much more fuel-efficient. That means motorists will remain reluctant to pay a fat premium for a green car.

Between now and 2025 regulators in Europe, America and elsewhere plan to impose ever greater curbs on cars' emissions of carbon dioxide (or higher fuel efficiency, which has much the same effect). This is forcing carmakers to invest both in developing electrics and hybrids and in making the conventional engine cleaner.

Ricardo, an engineering consultancy, and Sanford C. Bernstein, an investment bank, have crunched a bunch of numbers on the technology race between conventional and green vehicles. They conclude that petrol and diesel cars will keep closing the emissions gap (see chart), while hybrids and, especially, electrics, will be more expensive to own for years to come. The internal-combustion engine will still be king of the road in the early 2020s, when only a fifth of cars sold in Europe will be hybrid or electric. America's regulators are only now getting tough on fuel efficiency, so its cars are guzzling as much petrol as they did 20 years ago. However, in Europe, which got strict sooner and where fuel is heavily taxed, petrol and diesel vehicles have become much cleaner. The average new car sold in Britain now does 52.5 miles per gallon, up from 40.6mpg ten years ago. Even so, says Neville Jackson of Ricardo, there remains much scope for improvement: petrol and diesel cars still typically use less than a fifth of the energy stored in their fuel to turn the wheels. Plenty more miles can be squeezed out of each gallon. It is simply a matter of cost. To meet a series of deadlines to cut emissions, carmakers are putting into their cheaper models all sorts of gear hitherto mostly seen on pricey high-performance cars: turbochargers and superchargers (which mean the engine can be smaller and more fuel-efficient), fancy fuel-injection systems and valve trains; grilles with variable aerodynamics, and so on. Next year Ford will offer a new Focus compact car in Europe, with a one-litre, three-cylinder engine that performs as well as the 1.6 litre, four-cylinder engine it replaces, yet uses about 20% less fuel. Joe Bakaj, an engineer at Ford, says that even American buyers of the company's F-150 pickups, who would normally scoff at anything with less than a V8 engine, are switching to a new V6 version that performs at least as well but drinks less fuel.

The analysis by Ricardo and Bernstein shows the carmakers are in a tight spot: given motorists' aversion to the cost of electrics and hybrids, the quickest route towards meeting the deadlines for cutting emissions is to invest heavily in cleaning up their petrol and diesel cars. But to squeak past the finishing line they will still need a small proportion of hybrids and electrics. So they will have to keep spending on designing these, without their reaching a level of sales that will make them profitable.

Stefanie Lang of Bernstein says this will force carmakers to work together on developing new technology: Toyota, for example, has agreed to work with Ford on hybrid sport-utility vehicles and with BMW on both electric batteries and diesel engines. Carmakers are also dabbling in battery leasing (Renault) and car-sharing (Daimler) as they seek ways to persuade motorists that electric cars are affordable.

Hybrids and electrics will be a drag on carmakers' profits for years. But they are a useful marketing tool. GM has found that adding the hybrid Volt (pictured) to its model range is enticing into its showrooms the sort of young urban buyers who normally ignore Detroit-made cars. Many balk at the Volt's $32,000 price (after a generous government subsidy), but some end up driving away in a petrol-engined car like the Cruze, costing around half as much.