Nearly a year ago the city of Seattle started a field test of 14 Toyota Priuses converted to plug-in hybrid capability. At the time, the expectation was that the cars would top 100 mpg overall and 150 mpg in urban driving. The problem is that these PHEVs, like all other hybrids, are particularly sensitive to driving style. If you try to accelerate at any rate above the bare minimum the system won't stay in EV mode and the mileage improvement is slashed. After 17,636 miles, the 14 cars in the city motor pool are averaging only 51 mpg. That is certainly excellent mileage by any standard, but not that much better than a regular Prius.

The problem is the $10,000 cost of the extra lithium ion battery. At $4/gallon the fuel savings amount to only $200 annually. Seattle's experience is by no means unique. Google's highly publicized experiment with PHEVs is yielding similar results with its fleet of Priuses averaging 54.9 mpg and a pair of plug-in Ford Escapes getting 37 mpg. For all practical purposes, existing hybrid vehicles can't reasonably be driven as electric vehicles except at excruciatingly slow rates of acceleration. Given the cost differential, it seems like PHEV conversions may be an economic dead end (One of Toyota's PHEV Priuses is pictured [ UPDATE: image removed at the request of Toyota PR). The Seattle and Google fleets feature vehicles converted by third parties).

[Source: Seattle Times]

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