Fuel mileage results - as promised several weeks ago.

Well, OK, this is not a scientific test. But I think it's more sound than some of the "I get about x mpg" statements I've heard on occasion.

I wanted to test for any difference in mpg when using 87, 89, and 93 octane gas. Why? Commuting about 160 mi. total every workday, I spend a bit of money on gas. And, I think, it puts me in a position to measure fuel economy.

Basic informtation:
'89 Volvo 245, 4-cyl, non-Turbo, auto tranny
Tires: 195-75-14 on steel wheels, cold pressure about 34-35 psi.
No roof rack, no roof holes (less air drag)
15W-40 dino oil. (Soon switching to 10W-30 dino)

My plan was to drive a week or more on each octane rating, noting the miles from my trip odometer and gas consumption from the fuel pump at every fill-up. I did my best to keep everything else the same, changing only the octane. I was mostly successful at being consistent. Details after the numbers below.

87 octane - 1716.1 mi. - 69.4 gal - 24.7 mpg
89 octane - 1666.6 mi. - 64.2 gal - 25.95 mpg
93 octane - 1160.8 mi. - 46.2 gal - 25.1 mpg

I used 87 octane gas first, then 93 (after several days driving in between to get all the lower-octane fuel out of the tank), and lastly 89, again after a few days changeover period.

I drove the same route most every day: 98% highway on the way to work, and about 80% highway coming home. I did continue the test on weekends, but they included no extremes of heavy city drives or long highway trips.

After running the 87 octane test, I removed the heated-air damper from the air intake box. This caused no change to intake air temperature as the metal hose to the exhaust manifold was removed some time ago. My intent was to eliminate any restriction to the air path in the box. As most of my driving is highway, at fairly steady speeds, I don't think this affected fuel economy one way or the other.

About 400 miles into the 1666-mi. run of 89 octane, I did a tuneup - new plugs (Bosch Platinum), Bosch cap and rotor. I just couldn't stand to let it go any longer; the previous tuneup (same parts selected) was 30K miles ago in September '04. I'm sure that this improved the results of the 89 octane test. Shoulda done this before I started!!

Why lower mpgs with the 93 octane than with 89??
Two contributing factors:
1) I did the tuneup during the early stages of the 89-octane test, after the 93.
2) Having 93 octane tempts the driver. The car definitely ran stronger than on the previous 87 octane, so I think I did use a heavier "foot" during that test. You pay for that at the fuel pump.

What are the effects of higher/lower octanes? One mechanic explained to me that the knock sensor causes the computer to retard the timing when it detects knock. I used to adjust timing with a dwell tach while manually turning the distributor (not on Volvos). You could easily hear the rpm's drop as you retarded the timing. I think that anything that retards timing will reduce power AND fuel economy at that moment, assuming all other things being equal. Anyway, with 87 octane, if you step hard on the pedal, relatively little happens. With 89 or higher octane, the engine rewards you better when you step on the pedal. That's what my backside reports, anyway.

Some of you must be wondering why I've been using 15W-40 oil. Several of the best local Volvo gurus told me to use it year 'round, quite a few years ago. It's served us well, "Sophie" is now nearly up to 400K miles, but mostly retired. Needs a water pump and the new owner hasn't the money, but that's another story. I read (and recommend) a detailed discussion of motor oil at htp://www.lincolnsonline.com/article105.html
which has convinced me to try a thinner oil with a slightly narrower viscosity range.

If there's any discussion on this, and if anyone seems to care, I might post my calculating spreadsheets where you can see them. Excel files.