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.

I have an 01 V70, but very basic and similar to your machinery. I run Mobil almost exclusively, and get about 30 mph on my daily commute at very slow speeds. I avoid Irving for political reasons, they are the Walmart of oil companies. Consider regional variations in gasoline formulation. Last fall we took Jr to college in Minnesota, a 3400 mile round trip. Started out on a tank of Maine summer formula, that ran through at about 28. Car was stuffed to the roof rails. As we made westerly progress, the mileage improved. Maybe due to extended highway miles, but most likely due to different fuel formula. I was told that Minnesota has 10% ethanol, and that I would get lower mileage. I didn't. The best gasoline was the last tank, purchased outside Montreal, PQ. It ran through at 34mpg, and that was mostly on two-lanes across Vermont and the Kangamangus Highway in New Hampshire.
--
01 V70 2.4 M5 and a Mini

Thanks for the response. I don't drive beyond New England often enough to get meaningful information on other regions. Seasonal variations are something I am tracking but didn't mention. Summer mileage is higher than winter mileage, not surprisingly. I don't have enough data yet to break it down further.

Mobil was a surprise. It was my go-to brand for 20 years on previous cars, but I never really compared it in a systematic way until now. I will probably buy it occasionally in the future and continue to track. Maybe there's a bias in my data that will average away over time.

The two things I can say with a bit of confidence: octane is not a significant factor in fuel economy; bargain no-name brands are not a bargain.

I buy the Mobil because many of the local stations are owned by a man who helped me out years ago when times were tough. What goes around comes around. Also, I found that the previous Volvo ran much better on Mobil than on anything else, bargain brand or national. My V70 runs much better on 93 octane, as specified in the owner manual, and Mobil claims to have a cleaning agent. I experienced none of the throttle body fouling problems that many owners report.

My boycot of Irving is personal, political, and one of the few actions of my social conscience. Irving came south across the border, buying up small heating oil companies and building gas stations in small towns. Ran the competion out of business and jacked up their price. As I said, the Walmart of oil companies. When the senior Mr Irving died, he left his business to his sons on the condition that they never live in Canada. Odd, since Irving is the biggest employer in New Brunswick. Not odd at all, since the Irvings save a lot of money by not paying Canadian taxes on their personal property. There are several mansions on small islands just off Hamilton, Bermuda. No income tax in Bermuda, eh?
--
01 V70 2.4 M5 and a Mini

Great data - keep up the good work !

Sunoco will be an interesting data point since it has a high ethanol content - depending on the season and altitude.

MAT

Thanks for the encouragement. Sunoco is in my future if I can find a few stations that are on my normal routes. Highways in New England are generally sea level up to about 1000 feet at most in New Hampshire/Vermont. So most of my driving would be at sea level.