The government has a useful fuel economy website.

It allows you to check out the fuel economy for any vehicle you own, and if you personalize it with your actual miles driven and fuel prices in your area, it will calculate your annual fuel cost. I calculated this manually last month based on observed data and the website is pretty much spot-on. I probably do 75% highway driving, 25% city driving since I'm 12 miles down the highway from Charlottesville, but once in town I can run my errands pretty efficiently so I'm not driving all over once I'm there. I assumed that a hybrid wouldn't be particularly beneficial for me since they're mostly good for city driving.

Or so I thought! Apparently, they get pretty good highway mileage also. But an interesting data point on the site might contradict the official numbers and confirm my suspicion. Below the "rated" mileage rates is a section summarizing what people have submitted as their actual observed mileage costs. The Lexus ranged from 21-29 mpg, the Ranger from 21-27 mpg, but the Prius ranged from 32-72 mpg. Whoa. That's a big difference.

Terry had previously worked out that gas would have to be over $7 per gallon to warrant trading in a regular car for a hybrid. That sounds about right to me.

The fuel economy website will be useful if you have multiple cars and want to know which is most cost-effective to drive (although you've probably already figured it out), but it's probably most useful for people who are already looking to buy a car. They can enter their actual miles driven, plus city/hwy percentage, and find out in advance how different makes and models will work out for them.