Is it assur (forbidden) to drive a non-hybrid vehicle if you can afford a hybrid vehicle?
The Rabbinical Council of America actually called for its rabbis to:
And that rabbis should encourage congregants to do the same.
But I think "prohibited to drive anything non-hybrid" is a strong word.
While most (all?) of us would agree that consuming less gasoline is better, where would we draw a line at a prohibition? "Prohibited to drive if you could walk? Prohibited to drive a four-seater hybrid if you could afford a more-efficient two-seater hybrid?" Why specifically hybrids? What if solar-power is better?
While we have a religious value here of conservation, it's hard to automatically turn that into a bona-fide prohibition. ("Bal tashchis", "don't waste", for example, does not mean you can never, ever use disposable dishes. There are a few specific prohibitions of bal tashchis, and then a guiding religious principle.)
Now a community certainly has the right to enact a very specific prohibition based on a religious value, and that enactment would be binding in halacha on that community. But communal enactments are rare today, and I don't really see that happening here.
A more likely solution is to go to someone with power to make enactments, namely the government. The RCA thus supports increases in the government's requirements on vehicle efficiency (CAFE). If inefficient cars became illegal, "dina d'malchusa dina" ("the law of the kingdom is [torah] law") would most likely apply and make it prohibited Judaism-wise as well. (Though dina d'malchusa is a complex subject.)
In the meantime, we have a religious "should", not a religious "must"; often "shoulds" conflict with other "shoulds" (do I buy a regular car and give the rest of the money to charity instead?); the choice that best serves G-d for one person in his situation may be different than for someone else.
Why should it be? The only reason it should be obligated is because of "Veshomarta meod" and global warming will kill us all.
Even with all the evidence for global warming, and assuming changing MY car will make a difference, is there more evidence that changing to a hybrid will solve those problems than the amount of evidence that smoking is fatal in the 60's when Igros Moshe permitted smoking for the lack of evidence that smoking is fatal?