How to judge 'fuel economy' of plug-ins

What's the value of plugging in?

When tax credits for plug-ins become available, neither electric range nor fuel economy will matter, only battery capacity - a move that appears to strongly favor GM's Chevy Volt versus the Toyota Prius.

However, when it comes to the EPA and CAFE, it's fuel economy that will matter. And that is the standard that Congress will judge automaker fleet fuel economy.

So why the double standard?

Finish: Plug-in Ratings: Both Congress and the EPA confused