When Consumer Reports tested out the Hymotion L5 plug-in Toyota Prius conversion earlier this year, they weren't overwhelmed with the results. Sure, the mileage they observed was boosted to about 67 mpg over the first 35 miles of the drive, but that didn't match the claims of 100 mpg (or more) that Hymotion and A123 Systems make about their product.

To be fair to Hymotion, their qualifies mileage talk about their plug-in Prius MPG "that can achieve up to 100 mpg for 30-40 miles" this way:
Hymotion PHEV fuel economy is based on independent testing performed at Argonne National Labs and Idaho National Labs. Actual mileage will vary based on each individual's driving style, route, traffic, climate conditions, terrain and other factors.
Unfortunately for Hymotion, there are new results of tests done at the Idaho National Laboratory now available and they might make the company a little less eager to promote the work done by INL. The lab drove two groups of Prius test vehicles (one 40-car fleet and another 75-car fleet) from early 2008 until March 2009 for almost 500,000 miles and found that the average fuel economy tallied 46 and 49 mpg, respectively. As you might expect, driving style and the battery mode (charge sustaining vs. charge depeleting) had a big impact on the figures. You can view the result data in these PDFs: 1, 2. Add it all up, and it sounds like PHEV proponents might want to take former Tesla marketing boss Darryl Siry's advice to electric vehicle manufacturers to heart.

What do you think – are plug-in hybrids the next big thing, an overhyped solution, or something in between? Drop us a line in the comments.

[Sources: Idaho National Laboratory; Hydrogen Car Revolution]