Small Turbines are less fuel efficient in direct drive applications do to the fact turbines in reality have two speeds, very fast and off (Jets use afterburners to increase speed in turbines but only for short bursts of speed, such after burners just go through fuel). Given these two speeds and that most cars end up idling their engines at stop signs and while on low speed streets, conventional piston engines, given the fact they can idle at low engine speeds, use less fuel in such applications then Gas Turbines.
In a hybrid the advantages of low fuel use by piston engines while idling disappears for all the engine is doing is charging the battery and the battery actual provides the electrical power needed to propel the car. Thus a gas turbines better fuel efficiency at its optimum operating speed can come into play no matter what the car itself is doing i.e. if the car is going slow on a low speed street but its battery needs charged the turbine can kick on and operate as its top engine speed and that operation will have no effect on how fast the car is going or is dependent on how fast the car is going (and please note when I am using the word "Speed" I am generally referring to how fast the engine is operating at NOT how fast the car is going unless the use clearly is in reference to the speed of the car itself).
Even in the Prius with its conventional piston engine the engine cuts off once the battery is charged and you go by battery supplied electrical power till additional power is needed, then the piston engines restarts. With Hybrids a turbine's superior efficiency when operating at speed will increase fuel efficiency of the car over a piston engine.
I first read of a proposed Gas Turbine Hybrid in Scientific American in the early 1990s (It was attached to an article about the Peak Oil which the article said would occur around 2010).
Please note I am NOT an advocate of Hybrids, I believe they are a blind alley on the way to better fuel economy. Hybrids only exists do to the fact most people are NOT willing to down grade the performance of their car to improve fuel economy. In the 1970s I remember reading the Auto expert in Popular Mechanic or Popular Science (I can NOT remember which) who said he could NOT see why anyone actually needed a car with an engine larger then 2.5 liters (and that was in the days where most US made cars had 5.0 liter or larger V-8s, through by the mid 1970s even US car makers were downsizing engine size). Today, why do we need a car that goes 105 mph? Yes, some areas out west are miles and miles of miles and miles but even Montana had to pass a speed limit, furthermore as you double speed, you square (and sometime cube) the fuel usage. For example people will use twice as much fuel at 60 mph that they use at 40, everything being the same (i.e. in a car design to operate at 60 mph). Al Gore's son when he are chased by the Police in early 2008 had his Prius going 105 mph so such top speed can be achieved but other then running from the Police why do you need to be able to go 105 mph? Police, Fire, Ambulances and other emergency vehicles can be designed to go that fast but why should a car whose existence is to get its owner to and from work?
Thus I am more an advocate of the "City Car" concept then the hybrids. The VW Lupo was the classic "City Car". The Lupo did 78 mpg (Through that is NOT an EPA calculation do to the fact the Lupo was Never sold in the US). The Lupo did this by using a very small engine and having an electrical system that turned on and off that engine when it was not needed. Other power hogging devices were NOT put in the car (Power windows for example) or replaced by much high price mechanical systems that did not need power (An Advance Rack and Pinion Steering instead of power assisted steering). The engine was a 1.0 liter diesel tied in with a Automatic transmission (With manual override). In addition the car used a lot of light-weight aluminum and magnesium alloys to minimize weight and thus reduce fuel usage. VW stop making it in 2005 for all of the above made the car expensive to produce and the City Car Market is known to be very price sensitive (And thus was replaced by the less efficient but cheaper to make VW Fox).
The Daimler SMART car is another example of the City Car, again reduced top speed capacity do to use of a very small engines (Again like the Lupo a 1.0 liter engine). The SMART car does NOT do as while on the EPA tests as does the Prius for the SMART is not really designed for operating at 50 mph (Thorough even the SMART car can get to 90 mph on a flat road).
The Classic examples are the Japanese Kei Cars, these have engines SMALLER the 630 cc(.63 liter) but like the Lupo NOT imported into the US (Through at one time Nissan was suppose to do so in 2009, but I have not heard of that plan since mid 2008 when gasoline prices started to fall). These all promise to get better then the 78 mpg of the Lupo but as top operating speeds of less then 50 mph.
Now some people will say such cars can NOT operate on US Roads, but the laws that cover those roads (Except limited access highways like the Interstate highway system) not only permit such cars but permit bicycle, horse drawn and even people drawn carts (The police don't like them for people complain that they slow down automobiles, but unless it is a road that was purchased and built for exclusive use of cars, such as the Interstate Highway System, such man and animal powered vehicles are legal, no law says they are legal for the no one wants to address the issue for the courts have long held that to ban them you have to provide an alternative at a similar price, i.e. free and the State and Local Government do not want to provide such free transportation so they ignore the "problems" caused by Bicycles and hand and animal powered vehicles on roads originally built for such vehicles, which are most roads that predate the adoption of limited access highways in the 1930s).
Just a comment that the future of transportation, given the expectation that over the next ten years gasoline prices will go up, the best way to use oil efficiently is to cut down the top speed of automobiles and that can be done by restricting how large an engine a person can buy in a car for his personal use.
Now, the price of gasoline is expected to go down the next six months to a year, do to the drop in usage do to the recession, but that appears to be only a temporary reduction, the over all trend is upward and once the economy re-starts I expect to see the price of oil get back to what it was in the spring of 2008.
More on the Lupo:
http://en.wikipedia.org/wiki/Volkswagen_LupoMore on the SMART car:
http://www.smartusa.com/http://en.wikipedia.org/wiki/Smart_(automobile)
On Kei Cars:
http://en.wikipedia.org/wiki/Kei_carhttp://wikicars.org/en/Kei_carhttp://www.autoblog.com/tag/kei+car/Please note the above blog shows a 1.5 liter engine Nissan that it calls Kei Car, but Kei Cars are NOT to have engines bigger then 630 cc (or .63 liter).
http://en.wikipedia.org/wiki/Kei_car