Here's another fascinating entry on the "miles per gallon illusion" from the energy policy blog, Master Resource. Excerpt:
"it saves more gas to coax people to upgrade from their 10 mpg vehicles into ones that get a still-paltry 15 mpg, than it does to coax people to give up their 25 mpg vehicles and get into Star Trek material that achieves 100 mpg."
Read the full post to get the math on this.