I have always compared how my cars run based on differing fuel ratings. MPGs, knocking, power output, engine load comparisons and overall feel of the car. Also keep in mind, I don't miss anything in maintenance needs. If something happens, its taken care of quick. I drive 75-100 miles a day depending on circumstances through the day. Minimum is 52 miles round trip just to and from the shop 6 days a week.
1996 Cavalier 2.2 Saginaw engine with 5MT. 9:1 compression. Hates regular gasoline. This car now has 211,xxx miles. The EPA rating on this car was like 22-25/32-37. On regular 87 octane, that's about what it got, avg 22-25 mpg between fill ups. Sometimes worse. Midgrade, mpg went up a bit, load dropped, the engine reacted better. High octane, 93, the mpg jumped, load dropped again, engine loves it. I get 33 mpg between fill ups with a mix of highway and city driving. When I drive it across the country, SoCal from Tx and back, avg 45 mpg between fill ups cruising on the highway. It also still has the original CAT.
99 Outback 2.5 DOHC automatic. 9.7:1 compression. Would run good on 87, better with 93. This one it was more the way the engine reacted to the fuel than a huge difference in mpg. It averaged 21 between fill ups with 87 and increased to avg 22-23 with 93. Knocking reduced, engine load dropped and it reacted better, mostly on the highway.
01 VDC 3.0 DOHC automatic. 10.7:1 Prior to the SC installation and remapping for boost and then E85, this car got 20 mpg on 87 and 24 on 93. Again, the engine reacted better, load decreased and it was noticeable. Cruising on long drives, the longest from Austin to Padre Island last summer, the mpg increased a couple more points to 26. (side note: We did 3 remaps to the ROM just weeks before the SC install. After Ed's 2nd ROM revision altering the fuel/ignition map, my fuel mileage went from 24 to 27 mpg with improved performance and reaction. Running 93)
Every car I have owned for the last 27 years has performed better on higher octane fuels with exception of a Datsun 210 that didn't care one way or the other.
I don't care what articles say one way or the other and it doesn't matter how the fuel companies blend the fuel or process it. I know what I see as a result when I test it out. If I cared what was posted in articles, I wouldn't have modified the VDC because the articles said it was detrimental and wouldn't work. Professional builders told me it wouldn't work. Programmers told me what I intended was impossible.
So, try it yourself and see. Here is a link on how to do a detailed test in fuel usage comparison:
Fuel Octane Selection
Look it over. Go through the exercise if you like, only I would suggest you do a test now with the cooler weather, then again in the summer and average the numbers. Find what your car performs best on and go with that.
Some of my customers have done the same type of evaluation. Some run 89, others 93. In Subarus. Everyone drives differently and that would also determine your outcome.
