A Hummer that gets about 14 miles per gallon is clearly not as energy efficient as a hybrid Prius, obtaining 47 miles per gallon of gas. But when it comes to manufacturing, a hybrid Prius during the production emits far more carbon, thanks largely to its nickel based battery. It quirky catches up and surpasses in greenness the Hummer though thanks to the disarranging MPG rates. But buy a decade old used Toyota Tercel, where the carbon footprint has been paid off by the previous owner, and compared to a new hybrid they would have to both drive 100,000 miles each just break even in total carbon emissions.
All of this is based the current state of automotive technology. In ten years this could and likely will change dramatically. In the past couple years, the carbon emissions required to produce a nickel battery for a hybrid car has decreased slowly, and scientist predict it will continue to decrease as the years continue to pass. In addition, new technologies like the hybrid and electric cars require massive upfront investment and production. If the trend of them becoming more popular continues, the technology will be more common.
A practical view can change everything; a used 1994 Geo Metro XFi is so energy efficient (49MPG) that a Prius could never possibly catch up. But it has no AC or air bags. In a traffic accident—good luck.