Don't forget diesel. You can run biodiesel, which is made from deglycerinzed vegetable oil. Friendliest powerplant we've got at the moment. When the Subaru diesel comes out, I'm planning to seriously look into it. (though I love my current Subie, I'd rather be running biodiesel)

Plus the mileage from biodiesel is impressive. My sister in law gets 54 mpg biodiesel on the freeway in her VW Jetta.
I have a New Beetle diesel, and I traded my gas Beetle for the diesel so I could make my own biodiesel (and diesel was cheaper than unleaded at the time). But it's not as simple as just deglycinerizing the vegetable oil. (Although some people do run straight vegetable oil.) After studying the subject thoroughly, I'm not sure I want to go to the trouble. Plus, the workshop where we would make it is full of shtuff that we haven't cleaned up. IOW, I'm procrastinating (for two years!).

Most people now-a-days, if they live in the NE especially, will consider biodiesel to be what you can buy at the pump in the form of 85% soybean/15% petro deisel. I get 40 miles a gallon in my Beetle on the highway on petro diesel. (Your SIL's car is probably a stick, and she likely drives the speed limit or slower.) When I was in Brattleboro, VT last summer, right after Hurricane Katrina, I paid $3.09/gal for 85/15 biodiesel from a gas station there. I did not get better mileage on that tank.

Right after that, diesel crept up to the price of premium unleaded, and it has never gone back down. The going price in my town is $2.93 for regular no-lead and $3.09 for diesel (ironically what I paid in Brattleboro for biodiesel). I have taken measures so that I just don't drive much, but when I do, I'm still getting 40 miles per gallon!

The only benefit I would get from making my own biodiesel is that I will likely get the veg oil for free, even though there is the initial investment of equipment to process it (less than $200). Mathematically, it seems worth it, but time-wise, I'm not so sure, as little as I try to drive. If the biodiesel were available at the pump here, I'd buy it, though, because of environmental/political concerns. That's not likely to happen, living this close to oil-rich Oklahoma.

Karen