The Union of Concerned Scientist, a non-profit science advocacy group in the U.S., has released its second annual Hybrid Scorecard. The organisation criticised hybrid car manufacturers for not delivering on the technology’s promise, for placing performance above fuel efficiency and environmental benefit and burdening hybrids with features that put them out of the price range of average Americans.

A total of 34 hybrid vehicles from the 2011 to 2012 model year were tested for their fuel efficiency, environmental impact, value-for-money and their so-called “forced-feature premium”. The latter is features that don’t improve fuel efficiency but probably make the cars more appealing to money-conscious consumers.

Of those hybrids tested, only 13 cut greenhouse emissions by more than 25% compared to their petrol engined cousins. The worst offender was the Volkswagen Touareg Hybrid, which comparatively cut emissions by a paltry 10%. As is to be expected, the Toyota Prius retained its top non-luxury hybrid crown with 40% less greenhouse emissions that it’s nearest petrol engined equivalent.

The Lincoln MKZ Hybrid and Lexus CT200h also scored highly, as did the hybrid Ford Fusion, Honda Civic and Toyota Highlander. Nine of the top ten achieved better than 30 mpg and were rated as “above average” for environmental impact and value-for-money.

The Chevy Volt, which was conspicuous by its absence, could not be tested as the UCC lacks the proper metrics to determine the environmental impact of plug-in hybrid vehicles at this time. The organisation is said to be working on said tools for the next Hybrid Scorecard in 2012.

Here’s what the UCS had to say about the Volt’s absence:

“While the Chevy Volt, a plug-in hybrid, does have a conventional model against which to compare it (the Chevy Cruze), there are a variety of factors that affect a plug-in hybrid’s environmental performance. For example, the resources used for generating electricity (to charge the vehicle) vary widely in their environmental impact; electricity generated from fossil fuels such as coal and natural gas have higher global warming and smog-forming emissions than electricity generated from renewable resources such as wind and solar.

Driving habits also affect a plug-in hybrid’s emissions performance. We are developing a new tool that will allow us to give a more accurate environmental assessment of the Chevy Volt and other plug-in vehicles, and hope to have that available to consumers soon.”