If it says premium only then you should plan on putting in premium only
going with a lower octane causes the engine to misfire which can lead to damage... technically modern cars can get away with running whatever octane level you put in it because it can sense these misfires and adjust accordingly... however you're making the car run in a way that it was not designed to do just because you're a cheap bastard (IMG:
style_emoticons/default/tongue.gif) you'll lose power, and if the dealer is an ass they'll void your warranty for using the wrong fuel.
the only time I might consider using the wrong gas is if you don't really care about the car. I drive a 240sx it recommends premium and I always put in premium. I know quite a few people who use regular with no problems at all, however these people are also driving 15 year old cars worth ~$4K and looking for an excuse to replace the engine... so consider that.
Don't just think premium= more expensive... consider the differences in the fuel economy. Figure out how how much 100 miles in a tC will cost you vs how much 100 miles in an SI will cost you figuring in the price difference between premium and regular.
Also consider the difference in price you're paying for the car... how many miles do you plan on putting on in a year... how many years will it take to make up the difference in the price of gas?
Technically speaking, as the price of gas goes up, the difference between regular and premium becomes less significant because the difference in price typically doesn't move as a percentage. Premium usually costs 10-20 cents more per gallon whether regular is $1 or $4... that means if both cars get the same gas mileage and you drive 1000 miles a month you'll only wind up spending an extra $240 a year on premium... If the SI gets better mileage, then the tC then you might even be spending less on gas overall even though you're buying more expensive gas.
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As for the 6 speed vs 5 speed, it really comes down to how it's geared... for instance the 5 speed WRX vs the 6 speed STi... the top gears are pretty much the same between the two cars, but the STi has some what of an extra gear shoved in between 1st and 2nd when compared to the WRX... the result is that the STi actually has a slower 0-60 times because it requires 3 shifts instead of 2 and typically worse Autocross times because you're typically shifting between 3 gears instead of 2. It does better at the drags, and other racing applications though.
Alternatively the Corvette Z06 has a 6 speed but the 6th gear is so tall that it's worthess for anything but low RPM highway cruising, this gear is essentially only in the car to help make it more fuel efficient. the 6 speed in my fiancee's Mini Cooper S is the same way.
also keep in mind that in general 6 speeds have more problems than 5 speeds, they're more complex and more moving parts = more potential for failure. Also with a 6 speed vs a 5 speed in the same "package" the 6 speed is almost always weaker because they have to take out some of the internal bracing to fit the extra components. That STi 6 speed breaks about 5 times more frequently than the regular WRX 5 speed because the case is weaker. Many people who do big power STi builds will actually "downgrade" the transmission for this reason.
Obviously you're not considering putting huge power through your drive train, and I wouldn't worry about transmission failure in a new car... but consider how a 6 speed is geared before you go on about "wanting a 6 speed"... and keep in mind that more gears does not necessarily mean it's better.
This post has been edited by twistedsymphony: May 18 2008, 11:49 PM
