I added some more analysis about the claimed 60mpg biodiesel Hummer. It would take a lot of increased efficiency for the modified engines to be that efficient as there is no streamlining or weight reduction.
It would be interesting and useful to see more independent testing of the fuel economy of the turbine/hybrid hummer. A 60 mpg hummer would need to have power sources that are about three times more efficient than a Toyota Prius which also gets about 60mpg. However, the Toyota Prius is about half of the weight at 2700lbs versus the Hummer at 4700lbs. Plus the Prius has 0.26 drag co-efficient while the Hummer has a drag coefficient of 0.5 to 0.57. About 60% of the power required to cruise at highway speeds is taken up overcoming air drag, and this increases very quickly at high speed. Perhaps the higher mpg figure is because the hydrogen that is added is not included in the calculation. It takes power to produce hydrogen.
However, the claims of getting one of the less fuel efficient models from 9mpg to 18mpg is more plausible. The Hummer H3 is rated at 16 mpg in the city cycle with both transmissions and 19 mpg (US) (14.7 L/100km) with the automatic or 20 mpg (US) (11.7 L/100km) with the manual on the highway. Increasing efficiency to the 32mpg range seems possible with an efficient diesel conversion.
Perhaps more fuel efficiency is coming from efficiently charging a smaller number and lighter batteries and capacitors for the electrical drive with a while driving basis with the turbine.