ZF's nine-speed (above) was the world's first. Now Ford and GM think they can do 10. Image via ZF.
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A BSFC map measuring the efficiency ranges of a Ford 2.0L Zetec engine.
Read it like a topographic map, the regions with lower numbers mean lower
fuel burn per unit of power (so independent of total power.) For this engine,
the most efficient range is between 1500 and 3000 RPM. Via EcoModder.
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So why not have infinite gears to keep the engine permanently in its most efficient zone? That's the idea behind CVTs or continuously-variable transmissions, which use a band running through a pair of sliding cones instead of gears to give an unlimited number of ratios. (Yes, the video above is from Nissan, but it does a great job of explaining how CVTs work, and why adding gear ratios can boost efficiency.)The only problem there is that many consumers don't like CVTs at all. Unlike a regular automatic transmission, CVTs don't shift, which means you never hear the "bwahhhhh *kck* bwahhhhh" of an engine revving through the gears as you get on the freeway. Unfounded or not, people also generally don't like the idea that their gears have been replaced by a chain spinning in some grooves. So even if they're potentially more efficient, CVTs weird people out.Which means automakers are left trying to shove more gears into their gearboxes. Ten years ago, a 10-speed car sounded insane, but now they're on the way, and eight- and nine-speed cars are relatively easy to find. Will we ever have a 15-speed? Well, maybe. Semi-trucks have up to 18 speeds in their all-important quest to always maximize efficiency, and if an automaker ever figures out how to fit that many gears into a car-sized transmission (a rather difficult task), I'm sure it will end up in showrooms.@derektmead