Should you trade in your old gas guzzler on a more fuel efficient vehicle? Well, Edmunds came up with this handy little calculator to help you decide. I wanted to see if it made sense for us to trade in our 1988 Volvo 240 grocery getter in on a new Prius. The calculator only works for cars back to 1990, so I had to use a 1990 instead. Ok, whatever. Anyway, I was told I'd get $317 in trade (considerably less than all that Swedish steel is worth in scrap) and that the old Volvo gets around 19 mpg (which is about right). I factored in that we only drive it about 500 miles per month. The calculator showed me that if we switched to the Prius, we'd save $66.63 in gas per month, and that at this rate it would take 348 months (or 29 years) to reach break even and begin saving on gas. Hmmm, I'm thinking that the $23k might be better spent on some more old Mopars...