For the first 5 years I lived in Minnesota, I didn't have a car. I moved from DC, where everyone uses public transporation, and it just didn't occur to me that not having one would be a problem. Luckily, I could walk or take the bus to do most errands while I was in college, and I could borrow a car in emergencies.
Eventually, I bought a tiny Honda Civic with a manual transmission. Doesn't really get much more fuel efficient than that unless you're talking about a hybrid. And I didn't really drive it much--after 10 years, it had only 56,000 miles on it. I was living in St. Paul's Selby-Dale neighborhood at the time; I rode my bike to work most days, and it was easy to catch the bus or to walk if biking didn't make sense.
When we bought our house, I insisted that we could only look at houses within easy biking distance of work, and that we had to live on a bus line, too.
Expecting our first baby, riding the bus made me so nauseated that I kept having to get off and walk. And by the time the nausea subsided a few months later, Metro Transit had changed the bus schedule. I could still catch the bus, but even during rush hour it doesn't run very often, and missing one was a big pain. Plus, daycare wasn't on the same bus route as work, and just getting anywhere on the bus ate up all my time.
And then we were expecting baby #2, and the Civic's days were numbered. It was hard to wrestle our under-two into her car seat in the back of the two-door car, even without the big belly! And it would have been really difficult to manage two carseats back there. So we sold the Civic and bought a Honda CR-V. Not as fuel-efficient as I would like, but at least it's a low-emissions vehicle. We wanted a hybrid, but a) we couldn't afford it and b) even if we could have scraped together the money, the larger ones weren't available until July and that was too late for us.
So I still ride my bike to and from work when there's no snow or rain, but I end up driving WAY more than I'd like.

I love the light rail; I just wish it went places I need to go more often. If Saint Paul had a trolley or subway system, I'd use it every day.