TLDR; In my estimation, a new car never makes sense, from an environmental standpoint. And a hybrid never makes sense--it offers the worst of both worlds (all the pollution, manufacturing, and disposal footprint of internal combustion and electric).
All things considered, the fuel consumption of any vehicle is minor compared to the manufacturing, disposal, and maintenance footprints.
A used vehicle has the lowest manufacturing footprint, sometimes at the cost of greater maintenance footprint.
I have read (disputed) reports that the environmental footprint of a Prius is the same as that of a Hummer after 100,000 miles. True or not, it helps one think about putting fuel consumption into perspective of the larger picture.
Full disclosure: I have owned 6 diesel vehicles. I currently own an '86 TurboDiesel Jetta, and an '03 TDI Jetta. I have never owned a hybrid.
Without any specifics (because I don't know them):
A hybrid vehicle reduces emissions (although in the U.S., the most efficient hybrid is barely more efficient than the most efficient diesel vehicles, in Europe, there are even more efficient diesel vehicles). It increases manufacturing and disposal footprint.
Environmental factors which are roughly the same for both a used biodiesel and new hybrid vehicle:
- Engine coolant
- Motor oil
- Repairs for: Suspension, engine, windshield, etc
- Disposal footprint for any vehicle
Factors which are greater for a new hybrid:
- Manufacturing footprint of a new vehicle
- Additional manufacturing footprint for batteries, electric motor, etc
- Additional disposal footprint for batteries
Factors which are greater for a used biodiesel vehicle:
- (Possibly*) more fuel consumption, greater emissions
- Possibly greater fuel manufacturing cost
- Greater repair costs/footprint (compared to any new vehicle)
*My '03 TDI Jetta gets better fuel economy than all but the newest and smallest hybrids (and it's a lot more fun to drive). All biodiesel is not created equal. If you have a source of biodiesel recycled from used oil, the fuel refinement cost is practically nothing. If you are buying commercial biodiesel in the U.S., it is almost certainly made from soym and onventional soy farming is not sustainable. Although, the most recent numbers from the USDA suggest that soy biodiesel is more efficient than before (thanks @Nate). Although the USDA has a clear bias (an interest in promoting aggriculture), I have no specific reason to think their numbers are wrong.
My suggestion, in order of least environmental impact (assuming you are using the most fuel-efficient vehicle in each class, that will suit your needs):
- A bicycle
- A used all-electric vehicle**
- A used diesel vehicle, with recycled biodiesel or straight veg oil (SVO/WVO)
- A used diesel vehicle, with standard diesel or soy biodiesel
- A used gasoline vehicle
- A new all-electric vehicle**
- A new diesel vehicle
- A new gasoline vehicle
- A used hybrid***
- A new hybrid
**I don't actually know if an all-electric vehicle is better than internal combustion, but I suspect it is, because it reduces the mass of the vehicle, and many peripheral fluids (engine coolant, motor oil, etc).
***It's hard to rank hybrids accurately. Some biased sources claim they are devilspawn. Other sources, biased in the other direction, claim they're heaven-sent. The truth is probably somewhere in the middle. So take this absolute raking with a grain of salt.
Also note that even without biodiesel, a diesel vehicle is more sustainable than a gasoline vehicle. All other things equal, diesel gets roughly 33% better fuel economy than gasoline per fuel volume. Diesel refining is also, arguably, less harmful to the environment than gasoline. And lastly, a diesel engine is mechanically simpler than a gasoline engine, and thus will last longer (500k miles is common).
I suggest looking for a good quality used diesel vehicle. A 2000-2003 Jetta TDI is the ideal right now, easily over 50mpg highway. But even the newer Jettas still get excellent fuel economy, and the older ones can, too. My '86 gets about 40mpg. My '03 about 52mpg.