With gas prices hitting new highs I thought I would post a report on my real world experience with a Volvo s60 (03, Manual Trans, non-turbo, 195/65 oem tires). With 32,000 on the odom I've been tracking mpg since I first bought it. Average MPG for life of the car: 30.97, almost entirely highway driving w/lots of cruise control at 72 or below. The average octane rating of the 1000+ gallons used is 88.9. Here's a breakdown by octane and brand. Note that Irving is a Canadian refiner/marketer with stations in Maine, NH, and Vermont. I live in Boston.

87 octane (20 fill ups) 31.40 mpg
89 octane (44) 31.01 mpg
93 octane (11) 30.81 mpg

Irving (15) 31.97 mpg
Citgo (11) 31.70 mpg
Shell (15) 31.28 mpg
Gulf (9) 30.49 mpg
Discount (el cheapo) 30.31
Mobil (10) 30.02

Conclusions so far: (1) The differences in fuel economy among octanes are probably not statistically significant. (Probably none of this is statistically significant, but humor me.) (2) Aside from mpg, the car definitely doesn't like 87 octane. There's a slight but annoying hesitation on pressing the accelerator. Car runs fine at 89 octane or better, so that's what I use nowadays. (3) Differences among brands appear to be significant, so do your own experiments. (4) Discount gasolines, no names like Pace, Haffner, Prime, as well as Gulf and Hess on average get about 5% less than the name brands. Haffner seems to do better, but I don't have enough data points to verify. At $2 per gallon discounters have to be 10 cents cheaper to justify purchase. (5) Mobil, one of the high priced brands, appears to produce fuel economy in line with the discount names in my Volvo. I will avoid it in the future.

Final Notes: The car got about 27 mpg during the first 1000 miles. After that the mpg jumped. I did my first oil change at 3,000 and then switched to Castrol Syntec at about 6,000. Most fill ups are around 14.5 gallons, so there is about 20% mixing with the prior tank. I may try to get data points for Sunoco and Exxon.