I recently switched from 87 to 89 in my 06 Accord SE and I swear that the car idles much better with the 89. The engine just feels smoother when idleing with the AC on and just in general. Could it be due to the higher octane gas? Plus, I read somewhere here that Honda advocates for 89 in other countries.

 

No.
It is a waste of money and all in your mind.

Higher octane fuel reduces MPG. Octane is a rating for resistance to knock and is needed for higher compression motors.

 

Okay, well maybe not! Lol.

 

lol
It is a common misconception, so no worries! Maybe you used a better top tier gasoline like BP,Shell, or Mobil?

 

You know what? You're right. I switched from Arco to Shell. Uh, maybe that's why? Duh...

 

That could be it!

Many don't believe that any certain brand is better than another, but there is a difference and I ONLY use BP, Shell, or Mobil!

 

I notice that my car runs better and smoother with the B.P. Brand also

 

You mentioned Honda calls for higher "octane" in other countries; but you have to realize that they state the anti-knock index differently.

There's a dozen different ways to measure anti-knock ratings. They're all based on running a specific type of test engine at a very specific RPM, temperature, manifold vacuum, compression & spark timing. Some of the tests are meaningful for cars, some for recip airplane engines, or other things. For cars, it's MON & RON.

Using made-up numbers to illustrate, it's kinda like this:
MON (Motor Octane Number) = 84
RON (Research Octane Number) = 90
AON (Average Octane Number) = (MON+RON)/2 = 87

This gasoline blend would be labeled 87 AON (87 "anti-knock index") on USA gas pumps. The exact same gasoline would be labeled 90 RON in Europe.

Almost any blend of gasoline would measure differently by these 2 different test methods (MON & RON) with RON being higher. Some blends have greater sensitivity, meaning a wider spread between the 2 numbers.

Now, if you go back & try BP 87 gasoline, then try Arco 89, & do that over a period of many miles for each blend, you'll probably discover that the AKI rating printed on the pumps is not the complete & total story.

 

I'll be sticking with Shell from now on.