I'm currently driving a Suzuki SX4 2008. It gets (supposedly) 30 MPG (9.4 l/100km) on highways, though I suspect it's much lower now because of the beating I have given the poor car and lack of proper maintenance.
I regret the choice because I turned down a cheaper more fuel-efficient vehicle for it when I purchased it. At the time I had not anticipated how much driving I would be doing (a side effect of volunteering). I did over 15 hours of driving last week, almost exclusively highway. My usual week isn't quite that bad, but I do 30 minutes per day for work, and probably at least 3-5 hours a week for volunteer activities, likely more. A more fuel-efficient vehicle will definitely help. I'm considering either an electric or a tiny two-seater for my next vehicle, depending on how much they actual cost to purchase.
However, replacing a car has an environmental impact as well. The cost of building a new car releases a significant amount of pollution. I'm trying to figure out how the harm of purchasing a new car weighs against the harm of the pollution of an older, less fuel efficient, and poorly maintained vehicle. I figure keeping the old car would likely work out as environmentally better, but by how much?