The title of this article is misleading - you can't really calculate an mpg figure for a car which can run either on gasoline or electricity depending on which is available.
A figure of 250 mpg is not achievable by a pure gasoline vehicle the size of a prius - indeed, even a car the size of the VW lupo with a '100% efficient' engine wouldn't be able to get such a result. Drag and rolling resistance are the limits.
For the vast majority of people a pure electric car would be ideal - a range of 100 miles and an overnight charging regime would be a perfect match to most people's lifestyle. Of course, it doesn't have the flexibility of a pure liquid fuelled vehicle - so there will always be a niche market for pure liquid fuel or liquid fuel/electric hybrid vehicles.
In terms of overall efficiency, the gasoline engine is better than a grid-powered electric vehicle. Gas engines get about 30-35% efficiency (diesel about 35-40%).
Typical coal power stations get about 37%, grid distribution about 90% and battery charging about 95%, electric motor about 95%. (Overall efficiency about 30%).
So although power plants may be less efficient, they have the advantage that they can run on multiple fuels - coal, nat gas as well as be supplanted by otherwise unusable energy sources: nuclear, hydro, wind, etc. This flexibility is likely to be worth considerably more than a small loss of efficiency. Additionally, modern power plant technologies can improve efficiencies: e.g. 'clean coal' power plants are anticipated to acheive efficiencies of over 48% (although such plants are quite expensive - signficantly more so than recent nuke plants)
It is interesting to see BMW's stance on hydrogen. Essentially, they have announced that they see no market for fuel cell technology as an energy source for cars, and have chosen to develop their internal combustion engines for hydrogen. The main problems with fuel cells are extreme cost (and dependence on rare catalysts, which shows no sign of reducing) and low efficiency. BMW claims that their hydrogen ICE engines can exceed 50% efficiency - whereas most contemporary hydrogen fuel cells top out at about 60%. The 100-fold difference in price between a fuel cell power plant and an ICE power plant was obviously quite compelling for BMW.