Rickety math wouldn't be a problem if it weren't for the fact that we're trying to fashion an energy policy out of the dream of having a million electric and plug in vehicles on the road by 2015. (More math: today we have only 2000 capable of sustaining freeway speeds, according to cleantechblog. So we're looking at 5000 percent growth in six years. Talk about your hockey stick inflection point!)
But the other problem with plug in hybrids, in particular, is that their gas mileage depends heavily upon the behavior of the driver. As I wrote for Forbes, one person can coax more than 99 mpg out of a modified plug in Toyota Prius, while a pedal-to-the-metal type who forgets to plug the thing in at night will get less than 40 mpg. A study by Argonne National Lab found that driving style alone can reduce the electric range of a vehicle from 40 miles to 15. The numbers get even dicier from there out, because charging a plug in Prius in a state that's got a lot of coal fired electrical generators turns out to be not much better from a greenhouse gas emissions perspective than just using gasoline.
Plug in hybrids are a great idea for Detroit, and for the rest of us too, but they are not a silver bullet for the problems of energy security and greenhouse gas emissions. And they're really not solid enough to base an energy policy around.