I know popular wisdom here is premium is cheaper in the long run due to increased MPG. So I tried it this year on my usual 4K mile vacation. The past two years had both been 18.1 in a mix of everything from freeway at 80 speed limit to low range 4WD and I used mid grade. This year was similar in miles and conditions but with premium instead of mid grade I got 18.8! Cool, definite improvement. Except... That comes out to be just under 4% difference, which means if premium is 4% or less more expensive than mid, it makes sense. If not, it the numbers don't work.

To put it in dollars, if mid is $3.00, it's cheaper if premium is $3.12 or less. At $3.50 it's $3.64 or less and at $4.00 the magic number is $4.15 or under.

So, if it's the traditional dime a gallon difference between grades, go for it. If it's what is becoming the new norm of twenty cents a gallon difference, it doesn't pay to use premium. Recently I've seen some stations, mostly Mobil, that are 30 or 40 cents a gallon more in which case it definitely does not make sense to use premium.

I've seen comments about how premium is actually cheaper in the long run, but never with any numbers attached. I don't claim this is the ultimate scientific survey, but with 4K miles in each sampling it's not bad.

A few days ago I joined the same talk on other forum for my other car.
Over the years I have tried 89 vs 91 at Costco on my 94 Altima, 96 Maxima, 00 RX300 and 02 QX4. I didn't plan to do a thorough and accurate comparison, just did it whenever I liked to for quite a few times. I found there was nearly no difference in mpg, idle smoothness, etc that I could tell. Maybe I could with a professional meter of some sort. All comparison was based mainly on daily commute plus weekend random errand.

I have used nothing but regular in all of my paths ...

After 16 years of ownership, I have found no benefits to running premium gas in the Pathy.
I've used regular for the past 10 years, to no ill effects.

I've been running regular 87 octane fuel for 46k miles out of the 51k miles I have put on my QX4. No difference outside of my wallet being less unhappy
I noticed no change in mpg or seat of the pants acceleration.

I'm not trying to tell anyone what to run. I have seen multiple threads that claim premium pays for itself with increased MPG so I decided to try a semi scientific test, the results of which are in the first post. I didn't notice any increase in power, just somewhat improved MPG. Idle wasn't smoother, my teeth weren't whiter nor did the IRS send me a thank you letter for paying my taxes.