No bolt is being left untouched as carmakers in both the U.S. and Europe strive to develop cleaner and more fuel-efficient vehicles. Everything from weight-saving measures to aerodynamic tweaks and even downsized engines are being investigated in the hope that engineers can come up with fuel economy improvements. One area that has the potential for some of the biggest gains is gearbox technology.

In its quest for improved fuel economy, Ford has adopted a strategy of replacing its aging transmission designs with more efficient ones. The Blue Oval confirmed today that it would double the number of models equipped with its more fuel-efficient six-speed automatic transmission by the end of next year. By 2012, Ford expects 98% of its North American line-up will have the new gearbox, Reuters reports.

Ford has stated the six-speed auto offer 4 to 6% better fuel economy than the older four- and five-speed automatics. The new gearbox, dubbed 6F35, will debut in the 2009-model Ford Escape and Mercury Mariner (pictured) crossovers and will also appear in several Mazda models.

Beyond the new automatics, Ford will also add its dual-clutch Powershift transmission to a number of North American models with the first cars to be equipped with the fuel-saving gearbox due next year. Powershift has already shown to improve fuel-economy by 10% in the European Focus range and similar savings are expected for American models.