An electrical engineer has added batteries to his Prius and gets up to 250 mpg. I'd bet they are discounting the fuel used for his additional batteries.

CNN Story

 

 

As I have posted elsewhere labeling such a plug-in as getting 250 mpg is a mis-statement of fact, IMHO. A better way of stating what such a car 'gets' would be:
50 mpg + 200 extra miles on initial charge. After the plugged in battery is exhausted, thsi car has to lug around a greater mass than a non-plug-in, and thus would get reduced mileage.

 

So... Does this mean that if someone drives less than 200 miles per day and doesn't drive fast, the car won't ever use any fuel? That would be cool if it worked out that way!

 

More like 20 miles per day, but yes basically no gasoline needs to be burned.

 

The EnergyCS Prius is not a full "gasoline-optional" hybrid. The "assist" range of the larger battery is 50-60 miles, during which range gasoline usage is dramatically reduced but not eliminated completely.

This is because at any speed above 34 mph the gasoline-engine must kick in (for technical reasons, not due to any limitation in battery or motor power output).

In the unlikely event that you needed to travel a long way with low acceleration and at all times below 34 mph, then it should be possible to travel about 40-50 miles before any gasoline is used in EV mode only. (after the battery is depleted the gasoline-engine must switch on, at which time the car reverts to behaving like a normal Prius).