All WITHOUT using electric drive. Hybrids only make sense if you want to keep present day performance capacity, if you reduce capacity (i.e. think in terms of actual highway speeds NOT top performance) smaller cars with small gasoline or Diesel Engines make more sense then Hybrids. The main reason is hauling the batteries around cost energy i.e. at some point you will hit a barrier, the more battery you install, takes more energy to move the car around then the energy being provided by the batteries (i.e. the cost of the hauling around the extra batteries exceed the energy provided by the extra batteries).
A further problem is that in areas were the climate drops below 32, the energy shortage ability of batteries decline radically at such temperatures. At that point the Prius uses very little of its battery, relying more on its gasoline engine to provide the electrical power for propulsion. In such areas today (and that is most of the US) adding an additional battery pack makes little sense (The Prius was designed in Japan to perform in areas much like Japan and most of the US, i.e. Maximum battery use even during cold weather).
The high mileage performed by these modified Priuses is limited to areas of relatively warm climates (i.e. Southern California, Southern Arizona, Southern New Mexico, Southern Texas and the American deep South) the same areas were electric cars make the most sense (i.e. minimal lost of electrical stored power in the batteries do to the lack of cold weather). Furthermore the weight to power of the Batteries and electric drive compared to a Gasoline engine, its drive train and gasoline is limited (This is why Hybrid were made, the gasoline engine provides the power needed by the electric drive train to a much higher extent then electric batteries can, provided you look at both system of about the same total weight).
Interesting concept but remember its limitations, i.e. NOT really usable outside of the areas mentioned above. I know California is the state with the Highest Population, Texas is Third, Florida is Fourth (New York is still holding onto Second place) but except for Florida, all of these states have areas where it gets cold (The Mountains for every state except Texas, The Northern Panhandle and the rest of North Texas in Texas).
VW 235 mpg car:
http://www.inhabitat.com/2008/05/27/transportation-tuesday-vws-235mpg-concept-coming-soon/http://www.greencar.com/features/vw-235-mpg-car/http://www.edmunds.com/insideline/do/News/articleId=120448And may be for sale in 2010:
http://en.wikipedia.org/wiki/Volkswagen_1-litre_carFor information on the Lupo 3l Car (The car that did 78 mpg and you could buy, but NOT in the US):
http://en.wikipedia.org/wiki/Volkswagen_Lupohttp://www.vwvortex.com/artman/publish/printer_319.shtmlThe Lupo was replaced by the VW Fox in 2005, more to do with the high cost of producing the Lupo then any other reason. VW basically decided people would prefer a less fuel efficient car at a much lower price then the relatively high price Lupo, even at 78 mpg. Please note the Lupo, its successor the Fox and it slightly larger car in the "City Car" market of Europe and Latin America are NOT imported into the US (VW did import a "Fox" in the early 1980s, but it was a version of the much larger VW Golf/Rabbit NOT the Fox that succeeded the Lupo).