I've always just put regular gas in my vehicles, never saw the need, never had one that called for higher grade. Some recent threads and my BIL talking about his results using premium while towing got me a little curious so decided now gas price has come back down I'd try different grades. The '02 Xterra is our DD so it made sense to use it and it's pretty gutless so maybe any difference would be noticeable. Premium where I get gas is 91 so I started with it. I can't say I noticed any difference in power, smoothness, starting, and the mpg for the tank was right in line with normal fill ups. I hand calculate every tank by habit. There was one thing that happened that indicated a difference. On a 4 lane bypass road that I drive 2-3 times a week every week and have hundreds of trips on it by now there's a 1 mile uphill grade, not steep, that every time @ 65 mph the Xterra will downshift around 3/4 of the way up. Every time. With the 91 gas it has not downshifted one time. I ran 2 tanks of 91 and it pulled the grade every time. I use cruise at 65 and it hasn't mattered time of day, temperature, wind direction. I then ran it low and filled up with 89 and saw the same results, nothing notable while driving but it pulled the grade over the top every time through 2 tanks and I'm into the 3rd tank now. Fuel mileage right in the range I'd see with 87.
My guess is the higher octane is holding timing and it's just enough to pull the grade. This to me is more than anecdotal, in 4 years of traveling this road in every condition the Xterra has always downshifted, now with both 89 & 91 it has not downshifted one time. Is this little thing enough to make me want to run 89 at 25 cents more per gallon? Likely not. On our next towing trip I'm going to run some tanks of 89 out of curiousity.