ZF CEO says building autos with more than nine speeds is an exercise in diminishing returns.

The CEO of German transmission maker ZF believes nine forward gears in automatic gearboxes are the practical limit. Stefan Sommer reasons building hardware with any more gears than that will make them too complex and not result in corresponding gains in fuel economy and performance.

"There is no hard line, but you have to consider the law of diminishing returns," The question is whether adding even more gears makes sense," Sommer told Automobilwoche Congress, according to Automotive News.

Of course, one must consider the source may be slightly biased, since ZF is working on a nine-speed of its own, which is due to appear in Acura and Honda models. Chrysler was also reported to get ZF's eight- and nine-speed transmissions, while both Ford and GM are working together on their own nine- and ten-speed transmissions. Hyundai, also, confirmed it's developing a 10-speed trans.

Compared to a six-speed auto, ZF's eight-speed auto produced since 2009 offers 11 percent better fuel economy, while the nine-speed is expected to be as much as 16 percent better, the company calculated. There is little room for improvement, however, especially considering the costs involved. Today's most efficient transmissions are only 11 percent behind a theoretically-perfect gearbox in terms of efficiency.

ZF will begin assembly of its nine-speed gearbox in the U.S. at a plant close to Greenville, South Carolina. Chrysler will assemble eight- and nine-speed units under license in Kokomo, Indiana.