upper 20's/low 30's
I said this to a friend last week and I'll repeat it here: The Prius and the Insight have been engineered to have extremely low rolling resistance and some of the lowest Cd's of any production car. Put these on ANY car and they will produce better mpg. IMO a gas powered (non-hybrid) car with the same specs would perform similarly. Thats why GM's hybrid Silverado only uses the batterys to power the car while at rest, and not electric motors to move the car.
The only place we should be looking is alternative fuels. We should be moving towards using E-85 for the near future. It's domestically produced, clean, higher compression ratio's can be used, more boost, more power, etc.