14 specially customized plug-in hybrid Toyota Priuses didn't do much better than standard Priuses in fuel efficiency. Google's own fleet of hybrids and plug-in hybrids (Ford Escapes) are only averaging 28.6 mpg while their pluggable versions of the Escape hybrd get 37.7 mpg for a 32% improvement. That doesn't sound great. (hat tip to futurepundit.com)
It looks to me like the main problem is that they're starting with a Prius or Escape. Both of these are parallel hybrids which use both the gasoline engine and the electric motor, even if you stay within the 30 mile range of the batteries. If you drive with a leadfoot or at highway speeds, the battery doesn't get used that much.
A series hybrid plug-in like the Chevy Volt has only an electric motor. It uses only the battery for the first 40 miles. 78% of commuters wouldn't use any gas at all. Combine that with 50 MPG (twice as large as the average US light vehicle) for the 20% of driving after the battery runs low, and overall fuel consumption would be reduced by about 87%* (for a 567% improvement in MPG!).
* A Volt would use about 20% as much fuel as a very efficient (35MPG) conventional car - that's 12.6% as much fuel as the average 22MPG car.
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