With 50 percent of their daily car trips no longer than three miles, Americans could easily make half of them on foot or on bikes, which would save the nation 24 billion gallons of gas a year, proportionally cut tailpipe emissions, and reduce overweight and obesity rates, especially among kids, said Missoula-based Adventure Cycling director Jim Sayer at his slide show called “How Bicycling Can Help Save the World (and Missoula),” an early part of the group’s campaign for a coast-to-coast bike network.