What do we lack now that we will have in 2030? Why can't you build this 150mpg cruiser in 2009?
I hope lightweight alloys and composites will become more affordable. I'm sure rolling resistance will continue to improve, and I look forward to rolling on future LRR tires. Just be aware that current solar car LRR tires may be too low traction for anything besides solar cars.
Aerodynamics is a mature field. The existence of concept cars from the 1950s with Cd as low as .14 indicates we have been capable of excellent aerodynamics since at least then. The development of computational fluid dynamics may have been the final advancement in aerodynamics research. If automotive Cds continue to decrease, it will not be due to emerging technologies, but to making aero a higher priority during the design process.
A four seat, probably four wheeled, "stretch-Aptera" concept would come close to the pinnacle of fuel efficiency achievable with current technology. Punching the numbers for an Aptera that put on 300lbs into the simulator at
Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com , we get an amazing 158mpg at 60mph steady-state cruise. EPA fuel economy would be much less, maybe 100mpg? If there were enough demand for it, it could sell for $30-40k.