The auto companies have spent years and billions of dollars cranking every last bit of efficiency they could out of an engine that won't cost a customer twenty thousand dollars.
That, and shaving weight off of cars without reducing safety or incorporating unaffordable exotic materials, which is one reason I've got a load of disintegrating plastic crap under my van's hood instead of brass and steel hardware, and a touchy aluminum block instead of cast iron, which I almost had to replace after an overheat.
We are almost at the efficiency limits right now of internal piston combustion, which is why hybrids are such a quantum leap, and enthusiastically being adopted by car companies. Billions are being spent on that tech, too (a lot of it in Japan) and the holy grail of the decent electric car is still out there. Part of the issue is getting lithium ion batteries that won't catch fire.
In short, this is not something new for car companies. For years they've been doing exactly what you suggest.
Current Events » in reply to re: Who buys the commuter car?
Efficiency