Of all the natural gas tractor trailers on the road, Clean Energy Fuels (CLNE 10.44%) fills up 90% of them. Problem is, this really only accounts for 1,700 vehicles in an industry that has 3.2 million trucks on the road. Did I say problem? I meant opportunity! In this video, Fool.com contributor Aimee Duffy takes a look at the types of vehicles Clean Energy serves, and what the potential is for capturing a bigger slice of the long-haul-trucking pie.