I know this argument comes up only, oh, every six hours here, but...
If the car manufacturer tells you to use 91 octane (or 90 octane, but that's another argument for another thread), why would you use 87 or 89?
Do you think they said, "Hey, let's tell our customers to use higher octane, because we want to give people a reason not to buy our car, and we want to see them pay more for gas"?
Numerous threads here have summed up the facts, which are supposedly that: a) if you use a higher octane it's okay (albeit a waste of your money) because the car's computer will adjust for it, but b) if you use a lower octane you will probably decrease your mileage, and who knows what you're doing to the engine which was not designed for it.
Your lower-octane gas is detonating too easily and too early. It will reduce your power and strain your engine parts.
If you drive a car for 15,000 miles a year, and the car averages 30 MPG, that's 500 gallons of gas yearly. The difference between 87 and 91 used to be around .20, but now it's usually around .25 or .30 even. Assuming a .30 difference, for 500 gallons you're paying about $150 a year more for premium (and that's assuming you drive 15k miles, and only get 30MPG). And that doesn't take into account the lost MPG you might be incurring by using 87.
You pay around $15-20k for a smart with options. Is it really worth $150 a year to use cheap gas? That's three dinners out. Or 7 or 8 DVDs. Or a night out at your local casino. Big deal.
I walked into my local kitchen store just now and blew $40 on stuff I really didn't need. Then I went to a gourmet supermarket and blew another $40. That was about six months' worth of "buying premium vs. buying regular", and I didn't even bat an eye. Why is it that seeing those extra dimes on the pump's price sign makes so many people go insane, LOL?