93 is probably excessive unless either you look for the last .1 sec on the
stopwatch or your engine is dirty inside and is therefore suffering from an
elevated octane requirement.

Your engine can adapt to a range of octane ratings, plus there are an awful
lot of variables (temp, altitude, humidity, engine condition, fuel composition
[incl energy content and cleaning capabilities], spark plugs, ignition timing,
rpm/gearing, etc, turbo boost, etc, etc) such that there is no one ideal answer.

At the low end of the range, the fuel's octane is so low that the engine cannot
adapt enough to be gentle enough on the fuel to prevent
pinging/knocking/predetonation. This is, for a clean engine, somewhere below
87 octane (85, perhaps?, depending on so many things...). (All numbers US
method of R+M/2) As octane increases, the engine's knock sensor will realize
that it can apply more spark advance (and/or boost) to extract a little more
power from every (other) stroke without the fuel getting upset.

As octane increases still further (beyond a certain point), the engine will be
unable to realize any further advantage whatsoever, regardless of how much you
paid for the fuel with the highest rating. It's only linear within a narrow
range. Further, some of the fuels with the highest ratings have sacrificed
energy density for combustion resistance, and will actually make less power in
your car's engine.

Nothing beats a trial like a failure.

But be warned: even after a successful trial (no decrease in mileage nor
noticeable performance fall-off with a less expensive fuel, or an increase in
performance/mileage with more expensive fuel) the difference (or lack thereof)
observed may prove to be fleeting; after a number of tankfuls your engine could
develop a different octane requirement. Since octane isn't free, a lower octane
requirement (cleaner engine) is a good thing.

The fuel that keeps your engine cleanest may not actually burn the best, either.
Thus, given a certain brand of fuel and particular engine, you may achieve best
results alternating fill-ups between, say (depending upon...), 89 and 91. It's
more art than science, but if you can find one fuel that keeps your engine clean
and thus satisfied with it and burns well and doesn't cost much, use it
(regardless of its octane rating!).

- Dave; '95 854T, 123K mi

The owners manuel says 91 octane. End of story :)

Ihsan - You should consider getting an owner's manual. It will show you some necessary stuff, like how to open the gas fill door if the lock fails, and a few other tips.

I use premium fuel in three of my Volvos because they run better and get better mileage on premium. One does not.

I reccomend that you try a few tankfuls of each grade and check your mileage. We note the odometer reading on the slip each time we buy gas. This makes it easy to check your MPG on a regular basis. I don't go crazy over changes in mileage, but I do watch trends. Two years ago we drove 4994 miles out to Saskatchewan and back in two weeks. When I plotted my mileage, one tankful was 5 MPG less. That tankful was regular gas I purchased by accident.

For instance, two weeks ago, my cruiser's gas mileage dropped from 10-11 to 5-6.
That car is going for a check-up this week!
--
2 8s & 2 7s 600,000 miles total

my 96 manual for my NA 850 says that 87 octane is the one to use.

Can you find 93 octane? The best I can find in CA is 91 octane. Anyway I have a 1995 and I ran 91 at first but since it is N/A I could not tell a difference when I switched to 87. My 1997 850R is strictly Premium only. Good Luck.

Tony