A worthy idea no doubt- I'd encourage you to do the math first though. It might be a good idea and it might not. From an economic angle, the cost of a different car might be able to buy you an awful lot of gasoline. From a eco/green angle, producing a new car may impact the environment more than driving your existing truck.
You've probably thought about it already, I am just hearing a lot of people saying they want a more fuel efficient car, and I'm starting to get suspicious toward some of it. I heard on NPR something about used Geo Metros selling for $7k, which sounds like folly to me.