I have a 70 mile round trip commute when I'm in the office. When the weather is bad or I'm on the road, I drive my Jeep (sometimes pulling a small enclosed utility trailer). But when I only have to commute to the office I drive an old 87 Sentra that I resurrected. Not only does it save a huge amount in fuel, but it saves the wear and tear on the Jeep. I average 35-40 MPG, plates are $13/year, and insurance is about $150/year. If I do hit a deer with it I'm not out much. But the fuel savings is significant.
I figure the Sentra costs about $0.05 per mile in fuel (~36 MPG). At 15K miles/year that's about $1000/year in fuel.
The Jeep costs about $0.12 per mile in fuel (~15 MPG). At 15K miles/year that's around $1800 in fuel. So the savings can be significant, but probably not enough to warrant purchasing a gas saving vehicle on credit. I wouldn't be without my Jeep, however it is nice to have the little econobox to simply commute with.