GM, Ford to jointly develop 10-speed transmissions

• Ford and GM jointly developing 9- and 10-speed transmissions
• They will manufacture gearboxes separately
• They collaborated on six-speeds last decade

The latest auto race isn't on a track. It's under the car, unseen, in the transmission.

Ford Motor and General Motors, the two biggest of the Detroit 3, said today they will jointly develop nine- and 10-speed automatic transmissions, which could save fuel and improve performance, particularly with smaller engines. Developing a new transmission is a complex and expensive project — joint development could save each company hundreds of millions of dollars.

The automakers say their engineers already are at work, but don't say when to expect the gearboxes, which will share parts.

Companies legally can develop hardware together as long as they part ways to manufacture and tune it, as GM and Ford say they will. Unaffiliated companies even have shared engines. Chrysler, Hyundai and Mitsubishi jointly developed engines that they all used from 2005 to 2009.

The gear-speed battles seem a bit like the race to field the most cup holders or the most LED lights. But packing as many ratios as possible into a vehicle's transmission case is about much more than bragging points.

Done correctly, a transmission with more speeds improves mileage, performance or both by allowing the engine to work more efficiently. How well a transmission works, however, depends not just on the number of gears but on the software programming that controls it.

Chrysler Group is putting the first mainstream nine-speed automatics into the Dodge Dart and coming 2014 Jeep Cherokee this year. And while seven- and eight-speed transmissions are common among luxury brands, Chrysler is a pioneer at the moment for using eight-speed boxes in its mainstream big cars and, this year, Ram pickups.

It wasn't long ago that six-speed automatics were the new thing. In fact, Ford and GM collaborated on shared six-speeds last decade. While both GM and Ford are trumpeting that as an example of their joint success, those six-speeds got off to a rocky start.

GM used them in its big crossover SUVs, such as the Chevrolet Traverse and GMC Acadia. But it had to have dealers reprogram the gearbox control software several times the first year because of complaints about poor shifting.

Ford used its version in the Edge crossover SUV, among other models, but then-partner Mazda declined to accept the box for its adaptation of the Edge, called CX-9.

To boost fuel economy, a many-speed gearbox can have several "overdrive" gears, which let the engine run very slowly even though the car is speeding down the highway. For maximum performance, a gearbox with more gears can be set up to try to always keep the engine revolutions at or near the spot where it makes the most power. The art is to get the best of both approaches and to do so without annoying pauses or jerks, whether shifting up or down.

Some car companies, such as Honda, Nissan and Subaru, are going a different route with automatics called CVTs (continuously variable-ratio automatic transmissions). They have a belt-drive configuration that uses variable-diameter pulleys to constantly adjust the ratio for what their designers believe is the correct setting for the situation.

Some drivers don't care for CVTs' gearless driving feel, however, so automakers program in modes that either mimic the feel of conventional multispeed automatics or let drivers manually pick from six or seven ratios.