I drive a delightful gas-guzzling SUV. I'm also a math nut. I took into account the cost of a new prius, a used Mazda3 and my current SUV which will be paid off in less than a year.

My current SUV won out over all of the above - at least until gas goes over $5 a gallon. I used Kelly Blue Book to estimate the trade in value of my current SUV. And then a generic payment calculator for new/used cars on the Toyota/Mazda websites. I didn't take into account the rate difference in insurance and taxes however so this may vary for people. Figured out about how many miles I drive in a week and then the current gas prices and then $5 a gallon prices and how much it cost me to drive each week. The used Mazda3 was cheaper still than a brand-new Prius.

Remember when it comes to saving money, it's just not the gas money also take into account that monthly bill for the car and the insurance & taxes. You may find its just cheaper to drive your current gas-guzzler into the ground before you buy a new gas sipper.

Yes my gas bill is cheaper with a Prius but my overall monthly car expenses would be so much higher.