You're concerned about the environment and about the amount of energy you personally consume. You are thinking about reducing your carbon footprint. You are serious about changing something about your behavior, and you want to do something bigger than running down to Home Depot to grab some compact flourescent light bulbs for the house. But you aren't sure what you should do ... and you need some guidance.
Here's the simple solution: sell your car and replace it with a car that gets better fuel mileage.
I did it almost exactly one year ago, and it has been a great decision. I now go around telling everyone to do the same thing, so I might as well tell you about it too.
In my case, I swapped my Nissan Pathfinder SE for a used Honda Accord. Just like that I went from driving a vehicle that averages 15mpg (city + highway combined) to one that averages 24mpg. If I drive the Honda 13k miles this year, I'll be saving 325 gallons of gas. At $3.00/gallon (NY has a pretty steep gas tax), that's $975 dollars in my pocket. That's a pretty significant chunk of change.
I also obviously reduced the amount of harmful emissions I was producing by driving my car. FuelEconomy.gov tells me I am expelling about 4.5 fewer tons per year by making the switch from SUV to sedan. That feels good.
I loved the Pathfinder, but I could no longer overlook the fact that it was a total gas hog. Long drives from Albany, NY to family in Boston, Long Island, New Jersey and Connecticut and to vacations in Montreal, Vermont, New Hampshire and Maine were becoming more and more expensive as gas prices hit $3.00/gallon territory.
Sure, I miss being perched higher up on the road and I miss the security that comes from shift on the fly 4 wheel drive during winter snowstorms. But let's face it - those are luxuries I can manage without. I hear a lot of people say they drive a big SUV because they need the space. I think that's BS. The Honda packs very well and has bigger trunk room than would appear. Other than moving furniture, the Honda meets my space needs really well.
So, if you want to reduce your energy use, save a substantial amount of money and do the right thing for the environment, replace your car. Very simple. If you're already in a small, fuel efficient car, kudos for choosing wisely and responsibly!