I was talking with a friend over an aggressive quesadilla the other night, and told him about my new job and it's long commute. I mentioned that my Honda Element gets about 19 miles to the gallon, and my wife's Ford Focus gets about 26 MPG. By driving it to work, I'll save about $250 - $300 a year.
He suggested we trade in the Focus & get a car with better mileage, like a used Scion xA, Honda Insight, or old Civic.
So, I looked into it.
I found a used Honda insight in my area for sale for about $9500, which is estimated to get 51 miles per gallon. I've driven one before, and thought it was not the safest car on the road, but I couldn't find a car with better mileage out there, so thought it would be good to test out the math on it. Presuming we could sell my wife's 2000 Focus for $4000 (a stretch), and we dicked the guy down to $9000, that would mean we'd pay $5000 to trade.
If you go to Edmonds.com's True Fuel Calculator, you can find out the how much money a vehicle will burn in gas in a year & the estimated cost. So, if gas is $2.90 a gallon, I would save about $368 a year in gas if I switched from our Focus to the Honda Insight. (presuming 7000 miles a year, as it is mostly a commuter car, and our road trip car would still be the Honda Element)
So, how long before we break even?
5000/368 = 13 1/2 years! Yikes!
Ok, what if gas goes up to $4.00 a gallon?
I'd save about 508 a year.
5000/508 = 9.8 years! This does not include any repairs needed to the car.
So if you really hate burning gas, go take a bus, bike or train. Or just walk.
If you are in the market right now for a used car, by all means, consider one that gets the best mileage possible, particularly if you drive more than we do.
If you are thinking about getting a new car, also consider how much energy & CO2 it took to make the damn thing. Here's a website with such information, but there may be better ones: Green Car Guide
But if you have a car that gets 26 MPG, and are not going more than 8000 miles a year in it, you should just keep it as absolutely as long as possible.