2008 Prius Plug-in Hybrid prototype
In a long term study, the Hymotion Plug-in Prius conversion returned worse than expected gas mileage.
The study was conducted by the Idaho National Laboratory, and 115 Hymotion converted Priuses were evaluated. They were divided into two separate groups and monitored over the course of a long term study in which the vehicles logged over 400,000 miles.
Group 1 of the study included 40 Hymotion Priuses with a combined mileage of 188,124. This group managed to eke out only 46 mpg over the course of the study. Group 2 of the study fared a little better. This group logged 296,558 miles and returned an average of 49 mpg. These two groups were monitored from early 2008 through March of 2009.
A new group of Hymotion Priuses is being followed now. Initial data from this group with over 300,000 miles in the books shows an average of 46 to 51 mpg.
The converted plug-in Priuses in the study actually returned lower fuel mileage numbers than the current Prius is rated by the EPA. Many have claimed that this type of plug-in hybrid is inefficient and any gain in fuel economy will be modest in real world conditions.
The ongoing studies from the Idaho National Laboratory appear to confirm these suspicions. Their results show either a decrease or no change in gas mileage at all. On average, no group returned better fuel economy than a standard Prius.
Hymotion is a company that offers a battery conversion kit for a standard Prius which turns it into a plug-in EV. According to Hymotion, Priuses equipped with their setup can return 100 + mpg for 30 to 40 miles on a full charge. In lab testing, this may be true, but real world examples show significantly less mpgs. Why? Real world conditions are not always favorable to saving battery energy. Hilly areas, lack of stopping and therefore no regenerative braking, etc... all negate the capable range under battery power alone leading to reduced mpgs.