With knock sensor technology you would probably "get away with it" but the
difference in cost over even a long trip is not a lot of money. Say you go
1000 miles and get 25 mpg, that's 40 gallons at 20 cents more per gallon for
an additional cost of $8 not even accounting for better gas mileage using
higher octane and better performance. Is $8 that big of a cost assuming you
can afford to take a trip in the first place?

That's a very bad ideal.  You can damage your motor.  I have a '04 WRX.  If
you check your manual it says wrx must run a min of 91 octane and the STI
must run a min of 93.  On all the WRX(sti) site people have tried to use
cheap gas in our WRX they all report a lose of on min 2 mpg and a lose in
power.  Also the knock sensor only hears a knock after it happens.  So you
will still damage the motor.  I live out in Cali and we only have 91 octane
gas I add octane booster to every tank full just to give it that extra
margin.  Turbo motors are very sensitive to quality of gas  since they force
hot air into the motor they need help to control knock under boost.  If you
use cheap gas all it can do is reduce timming and try to cope.  You are only
talking a differance $1.50 a tank full(15g tank) .  Is your car not worth an
extra $1.50 a tank.  Depending on how you drive you should be getting around
22 +mpg on the highway anyway.  I also have a '64 Pontiac Grand Prix that
has a 389 w/tri-power (three 2bbl carbs) that the manul says needs a min of
98 octane and recomends 100.  I would drive to the compton air and fill up
with low lead 100 octane air fuel.  At about $2.80 a gal for a 25 gal tank I
would spend about $70 every fill up.  I was my daily driver that got about
maybe 10 mpg and would have to fill up every week. Finaly the EPA cracked
down and I could not get the 100 octance.  So after have the car over 2yrs
and putting over 20k original(91k mi when I bought it now reading a little
over 110k). I pulled the motor and built it for unleaded and lower octane(91
oct and $5 a bottle oct booster).  I went through all that trouble because
when I bought that car I know what it needed.  So give your car what it
need.  Isnt you car worth $1.50 a tank full?

You'll probably end up getting worse gas mileage... I've found this to
be the case with cars that recommend higher octane fuel.

So you might hurt your engine without any savings to show for it.

Nick

The knock sensor will probably prevent any major damage from happening to
your engine, but you will probably get *worse* gas mileage. The knock sensor
is an inherantly passive device, meaning that it waits for a ping to
actually happen before cranking back the timings, it doesn't proactively try
to adjust the timings. It'll keep your engine safe, but it won't make it
efficient.

   Yousuf Khan

Okay, I'll ask the obvious question: why did you spend $30K+ on a
performance car like the STi if you don't want to spend the coin on premium
gas to protect your car's engine? The answer to your question is pretty
simple: use premium gas; don't screw around to save yourself a buck here and
there.

Jen

Ok, let me explain about forced induction (turbo or supercharging) and
how it relates to octane ratings.
First of all the octane rating of gas is it's resistance to what's
called detonation.  Higher rating=more resistance=better burn
When you're running any forced induction application this becomes
critical in that you start running a lot more air into the engine.
Remember, all that a turbo does is force more air into your engine so
that there is more air to burn.  Now if you run any gas with too low of
an octane rating you are basically now running too lean on the engine.
It's just as if the engine wasn't getting enough fuel. Hence detonation.
 Very bad juju.  Too much air+not enough/wrong fuel=BOOM!  Forget
knocking, we're talking about destroying your pistons, and blowing the
head on the motor.  Trust me, this information came very painfully to me.