The fuel efficiency of a hybrid (HEV), like Prius, seems disproportional to its cost. Is 50 mpg really as good as it gets? Hondas were getting close to that on the highway in the eighties. Seems to me auto manufacturers could do better if they were sincerely motivated.
But unless you want to straddle something, 50 mpg is as good as it gets–when it comes to internal combustion, that is. Right now internal combustion still has the considerable advantage of superior range over EVs. The plug-in hybrids (PHEV) combine the advantages of both. [link]
Ten to twenty percent of any vehicle’s carbon emissions is spent during its manufacture–one good reason to keep your old car. This figure is actually a little higher for hybrids and EVs, mainly because of the batteries. Lithium comes almost exclusively from China where subsequent pollution has damaged much agricultural land.
These batteries do have a very long lifespan, however–especially the more recent models. Somewhere between a hundred and two hundred thousand miles at least. And they are prized for their recyclability. Plus, the battery technology is improving. Prices are supposed to drop 50% over the next few years, and efficiency is going up 8% every year.
You could do a lot worse than a hybrid. However, the carbon and karma emissions of traveling could be much lower.