My car says 91 octane, so I fill up when the tank is 1/2 full, alternating between 89 and 93 octane.
I did a test of different octane ratings about a year and a half ago. I found that while l would save a bit of money (about one penny per mile) by using 87 octane, the performance was not nearly as good as when I use the recommended octane.
pullen - You should be okay on 87 octane, your car will compensate, but you may notice a little less acceleration or power.
Drive like normal. Just don’t try to pull a gigantic trailer over the Rockies. If your idea of “normal” driving is foot-to-the-floor off each stoplight, you’ll notice the car has a case of the slows. But you won’t hurt it.
At least assuming it was built after ~1990. When asking for specific “what should I do?” car advice, including at least a make and year is a good way to start.
Octane is mixed different depending on region and altitude. 91 or 92 Octane IS premium in Colorado. 94 Octane is premium at lower altitude.
My Dakota truck manual says 87 Octane. I use 87 Octane in the truck.
My G35 Coupe manual says 91 Octane is required. I use 92 or 93 because 91 is rare in these parts (DE/NJ/PA) on my commute.
My truck sat for six months with less than a quarter tank of gas one time, so as soon as I fired it up after some overdue repairs, I drove a half mile to the nearest gas station and put a full tank of 93 in it, because I have read that old gas loses octane, and I have no idea how bad that old gas really was.
Read the manual.
Good point. 2001 Lincoln Continental. Four-point-something V-8. My “normal” driving is pretty much what you’d expect of a 53 yr old man in a Lincoln.
Based on what I’m reading, the car probably never noticed it didn’t have enough octane.
Thanks for the replies, everyone; (Now get off my lawn:p)
I buy plus, because the manual says to buy plus…
Dodge Challenger RT (V8 hemi).
Same car here, and same year. Engine’s a 4.6 by the way. I usually use regular unleaded. The mileage only suffers a bit. I have all of the math justifying it based on the price of gasoline and the reported mileage using different grades. You can probably find it in a search. Oh, and yeah, US prices and US units at the time.
Of course now that the car is at 8000+ feet above sea level, I actually, really, truly do notice the difference between the 87 octane and the 91, and so use the 91. Because of many complex reasons, it only leaves the garage once a week anyway, and so extra pesos don’t really mean anything to me.
My spouses cars manual recommends plus for over 5000 feet so she buys that quite often.
I usually don’t bother with “premium” petrol for general driving, but if I know I’m going to be doing a lot of driving in a given week, then I might fill up with BP Ultimate, which is 98 Octane and, I have noticed, gives significantly greater mileage that the usual 91 Octane stuff.
And, in contrast, I sometimes use the E10 stuff (91 Octane with 10% ethanol) because it’s cheaper and still makes the car go, but I’m still wary enough of it not to use it as my “regular” petrol.
No snark intended. 
You also didn’t say what model of Nissan or Toyota. I thus assumed the most common models (Corolla, Camry), which use regular.
My 10 year old Corvette cries if I don’t give it higher octane. We try to treat the kids alike, but she is such a princess.
Both of our cars (Mazdaspeed 3 and a Mini Cooper) require premium. It works out to a couple dollars more per fill up, so it isn’t like we’ll go broke filling with premium.
I’d wait until the tank is 1/2 gone then fill it back up with premium.
You can also buy bottles of octane booster from most decent auto parts stores, but I’m not sure how effective it is. Might be a good solution while you’re waiting to use that first half tank. Ask a mechanic or one of the (knowledgeable) staff at the store, though, just to be on the safe side.
I run premium in my Sportster. It has a modified engine with high compression.
In my 15YO Chevy? Nah. I gas it up with whatever’s cheapest. Hubby’s 4YO BMW? Premium all the way. The owner’s manual recommends it, his mechanic backs this up. But as has already been mentioned, this is a classic case of “performance car” (his) vs non-performance car (mine).
Snake does.
“She needs premium, dude…PREMIUM!..DUDE!”
My Chrysler LHS and Jeep Grand Cherokee both said to use 89, so I usee 89.
I used regular 87 in my Chevrolet Caprice when I had it.
The only time I use something other than what is recommended is if one of the pumps is out of order, and all they have is the other grade of gasoline, or I’m stuck needing gas near the gas station that only has 85 or 93 gas.
When I’m doing a longer road trip, which stresses the engine more, I may go up on the octane. Usually, what I’m looking for is non-ethanol for longer trips, so that sometimes means going up on octane.
For the daily drive (city-driving mostly), I just use regular.
My chainsaw needs high octane, so it gets it!
Yes when needed.