I've read all of the threads that RobyJo mentioned, and I'm happy using 89 octane. I've run several tanks of 89 (and a couple of 87) with no problems or apparent degradation of performance or gas mileage. I will probably use 91 when I'm fully loaded or pulling a trailer, but otherwise, I don't see any advantage for 10% increase in operating cost. (I will also probably get a tank of 91 about every 5th fill up or whenever I find a good price.)
A recent thread addressed gas mileage experience using lower octaine . . .the mileage actually improved! And as opposed to Boston not finding 91 octaine, a station here in Connecticut ran out of Regular and was selling 91 octaine at the 89 price (twice). Well, folks, all of those low gas prices are beyond us now, we're going to get SUMMER prices now.
I usually burn 89 octane. Around here it is a 10% ethanol blend and is usually the most inexpensive. Once in a while, when the price is right, I have bought 91 octane. I don't notice any difference in fuel economy or power between grades. I have noticed minor differences in mileage between tanks but can't correlate it to any consistent factor.
In my area, there are 87, 91, and 93. well.. one gas station (Crown) is using 87, 91, 92... I always buy 93, because on certain day of week.. 93 octane is on sale, which is only 2c - 4c different from 91 octane.
I just got back from a trip to the Florida Keys (over 1,000 miles one way), and filled up at various places, buying mid grade (89 octane) and premium (mostly 93, but one was 92.5). I was a bit disappointed with the mileage, which was hovering around 21. It didn't seem to make any difference between midgrade or premium in general, but I got the best mileage from Shell premium, at just over 23, and this was driving with a combination of 80-85 MPH speeds and slow crawls through the horrible traffic! Someone else had posted better mileage with Shell midgrade, so I'll probably try that at some point, but Shell stations are not on my regular driving routes.
I've used 87 80% of the time with no problem for the past 14 months. Also no noticeable difference in performance or MPG. And now that we're approaching $1.70 a gallon for regular I sure as heck am not going to pay $1.90+ for premium.
I don't know if the MDX is a barometer of gasoline or not, but it seems that octane or retail purported grades don't matter from what I've experienced and read so far. I've also noticed the same with my motorcycle but that isn't relevant to our forum theme of the MDX.
I'm sticking with the most inexpensive fuel as I've done so far, unless other empirical information comes forward.
It makes me wonder if there really is any difference between grades, other than profit margins and cost. Perhaps there is someone here who is from the industry who can comment.
As I understand use of gas with improper(not what is specified for the particular engine) octane number(especially prolonged one) can cause some decrease in performance(worse mpg and acceleration etc.) and also damage to the engine ("premature detonation" - knocking etc.), but I doubt that this is true in the MDX case, since there is basically the same engine for the new Odyssey with no such requirement there. In my opinion they mentioned premium gas only for the sake of luxury.
On the other hand, as I bought my car and intend to keep it for a long time, I prefer to use premium gas, - I believe it'll help to keep the engine in a better shape longer. Also several people here mentioned they've experienced knocking when lower grade gas was used.
BTW, speaking of mpg, - by my observation AC takes more than 3mpg out of the mileage.