ONE of the conundrums of the car business is that five smaller Japanese firms continue to prosper alongside three giants, Toyota, Nissan and Honda. In theory, those in the second division—Mazda, Mitsubishi, Suzuki and Subaru—should long ago have merged with rivals at home or abroad, or fallen by the wayside. Daihatsu is already controlled by Toyota, which has a 51% stake in the firm. They all sell 1m-2m vehicles a year. Sergio Marchionne, boss of Fiat Chrysler, once said that 6m was the minimum required for carmakers to have a hope of turning a profit.

Mazda, an even smaller firm, gladly parted ways with Ford. The American carmaker began to wind down its stake in 2008 to raise cash and avoid bankruptcy. The two firms had worked together since 1979. Subaru, part of Fuji Heavy Industries, a conglomerate, reportedly chafes at the 16.5% stake that Toyota, the world's biggest carmaker, holds in it. One obvious solution for the sub-scale firms—mergers with the biggest three—seems a distant prospect.

It helps that all are making generous profits after years of losses. A weaker currency means they are well-nigh printing money, notes Max Warburton of Sanford C. Bernstein, an equity-research firm. Subaru and Mazda, the biggest exporters among the five, are enjoying record sales in North America. Subaru now outsells VW there. The Japanese small-fry are also more profitable than most firms in the industry.

That may not last. A succession of lean years means the smaller firms lack cash to invest heavily in new technology. But they are finding smart ways to turn weaknesses into strengths. The industry admires Mazda's decision some years ago not to develop costly hybrid or fully electric powertrains in favour of pioneering its “SkyActiv” technology, which greatly improves the efficiency of petrol and diesel engines.

SkyActiv is typically pragmatic engineering from the Japanese, and just what the market wants now, says Mr Warburton. In the coming years, however, meeting tighter emissions standards will require more capital spending. It may also be difficult for the smaller firms to invest in technologies that support autonomous driving, should it prove popular.

Another explanation for the small firms' endurance is the tacit support of Japan's government. In the case of Suzuki, Mitsubishi and Daihatsu this support is visible in the form of longstanding tax breaks to miniature “kei” cars. Much loved by women and the old, the tiny cars and trucks now account for around two-fifths of new-vehicle sales in Japan. Nissan and Honda manufacture kei cars, but the three smaller makers rely on them more.

The government now seems to have heeded the warnings of the largest carmakers, that making kei cars diverts attention and funds from the development of models with export potential. A decision earlier this year to raise taxes on the category bodes ill for their manufacturers. While Subaru and Mazda are successful outside Japan, and Suzuki is envied for making big profits selling small cars, the weakest of the second tier may soon face fresh difficulties.

The underlying problem faced by all Japan's carmakers is a declining and largely unprofitable home market, says John Harris, a consultant. Yet for Mitsubishi, probably the weakest of the five, the problem is eased by support from the huge industrial group it belongs to and Daihatsu will continue to enjoy Toyota's protection. With this level of help it is unsurprising that none of the small carmakers has yet run off the road.