On the road

We measured the Prius' gas mileage on our standard fuel-economy test cycle, which involves a mix of highway, rural, and city driving. While the converted Prius operates the electric motor and gas engine as needed, as in a standard Prius, it spends more time running on electricity and relies less on gas. Still, when running solely on electricity, any moderate acceleration or mild grade makes the gas engine kick in.

We could usually drive an average of 35 miles before the new battery was depleted; unlike the original battery, it can't be charged by the gas engine. After that, the plug-in Prius returns to its normal operation. But the extra weight of the new battery drops its gas mileage from 42 to 40 mpg overall and from 34 to 29 mpg in the city.

The 67 overall mpg we got during the first 35 miles is a 60 percent improvement over the original Prius Touring. But that's based only on measuring gasoline consumption. A full recharge took about 6 hours and consumed about 5 kWh of electricity. That's about 55 cents at the national average of about 11 cents per kWh. With help from the Argonne National Laboratory, we calculated that by adding this energy into the equation, overall fuel-economy is equivalent to 53 mpg.

If gas prices were to hit $4 per gallon again, the cost to operate the standard Prius Touring version would be about 10 cents per mile; the converted version, about 8 cents per mile.