According to CarTalk.com, I've been wasting money filling up my 2002 Mitsubishi Lancer ES on premium for 9.5 years. D'oh.
Here's something I never knew for sure: the fuel tank spec says it holds 13.2 gallons and supposedly it should get 23mpg in the city, or 303.6 miles to the tank. I must really beat the hell out of it because I get more like 220-240. Although... I also think there's a big fudge factor on the fuel gauge, because when it's whining about feeling empty, I generally can't put more than 11 gallons in. I think a couple times I put as much as 12, but the fuel gauge was so low I was sure I was running on fumes and in danger of getting stuck.