Kitsune - when I bought my Insight in 2003 I went through the financial analysis and figured that I wouldn't be saving any money (at least during the first five years while I was paying off the loan), unless gas went to $4/gallon. If I sold my truck (a Ford Ranger) the economics would have been positive at about $3/gallon, and I was planning to do that, but we have too many days where I wouldn't be able to get to work in the Insight. (I live in the foothills west of Denver at 7,800 feet.)
As you mentioned, the economic analysis really depends on what you're comparing. It might be tough to justify purchasing a new Prius versus keeping an existing SUV, but if your choice is purchase a new Prius vs. purchase a new SUV, the Prius will save money both in the purchase cost and in the operational costs. If you're putting 20,000 miles per year on a truck that gets 15 mpg at $3.30 per gallon, you'll spend $4400 per year on gas. If you drive the same mileage in a Prius getting 55 mpg, you'll spend $3200 per year less. If the incremental cost of the Prius over some period of time (say 10 years), including fuel and insurance costs, is less than the comparable costs in the SUV, then it makes economic sense.
But there might be other reasons, beyond economics, to switch to a small car. They're easier to drive, easier to park, and handle better. One thing that I like about the Insight is that it can go 600 to 800 miles on a tank of gas, so I spend less time at the gas station.
A Corvette driver once made a comment to me that the Insight wouldn't pay for itself, and I said "But the Corvette will?" I didn't understand his logic, because he didn't have any.
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