USA Today reportsthat Americans drove 30 billion fewer miles between November 2007 and April 2008 than they did during the same period the previous year. It's the biggest drop since the late '70s oil shortage.What the reporters don’t mention is the environmental impact of all of those miles not driven. The EPA has a helpful equation you can use to figure this out. And I’ve done the math for you: 30 billion fewer miles driven kept nearly 25 million pounds of carbon dioxide out of the atmosphere.(Here's the calculation: (VMT/passenger vehicle avg. MPG) x CO2 per gallon x (100/95) /1000 = lbs. of CO2. I used approximate MPG (23, instead of 23.9) and approximate CO2 (19 instead of 19.4). So, with the actual figures, it's:(30 billion vehicle miles/23 miles) x 19 x .001 = 24,782,609 lbs. of CO2)The reporters do put that 30 billion in perspective: