In U.S. alone, our cars' tailpipes spew some 1.2 billion metric tons of carbon dioxide per year. That's a big part of why our global climate is changing. But there are a lot of ways to cut back on that pollution without switching to alternative fuels or electric cars. That's because internal combustion engines, despite a century of tinkering, have a lot of room for improvement in efficiently converting gasoline to power. For instance, simply changing the action of intake and exhaust valves in a car while it is running could significantly improve gas mileage. And that's technology that exists today. Simple changes in our driving could help too. Turning off the engine when idling, making sure tires are properly inflated, coasting to red lights rather than slamming on the brakes and slowly accelerating from a stop - all those can boost fuel efficiency as evidenced by hyper milers, those who have mastered the art of traveling hundreds of miles on a few gallons of gas. Ultimately, hybrid vehicles and electric cars whether powered by batteries or fuel cells will deliver even more environmental rewards. But until they are widely available, simple changes to the internal combustion engine - and the way we drive could as much as double gas mileage.