Okay, this is something I've been experimenting with for he last couple of months. When I fill up with the regular grade gas, I get pretty decent mileage (about 17.5), but my idle seems to be a little rough. When I use middle grade or premium I get a much smoother idle but my mileage drops by about 2 to 2.5 miles per gallon. Weird? I figured a smoother idle would yeild a higher mpg number.

 

That is kind of weird.. are you changing the way you drive when you put the higher grade fuel in? ie: stomping it off the line.. higher speeds, etc?

Also, how many miles are on your motor? You might stand to gain a smoother idle with the 87 grade fuel if you cleaned everything out.. like with a can of seafoam or something.. get the gunk out of your motor.

 

It really depends on how long you are running one octane rating. It can sometimes take a few tanks to get the most out of an octane change. This is especially true if you fill up at around a 1/2 tank or more.

And before the PCM can adjust to the new octane rating, it should be expected to have a little bit of deminished performance. The higher the octane rating, the harder the fuel/air mixture is to ignite. Timing will advance as the PCM adjusts, taking more advantage of the higher octane fuel.

 

I guess I'll just have to give it a little more time then with the changes. So far the middle grade has given me the best idle and gas mileage mix. The premium seems to be a little rougher than the middle grade, but like you've said, It may take a couple of tanks. I usually fill up when the needle hits midway between 1/4 and empty. I don't punch it off the line on my truck, however, my more recent driving hasn't included quite as much highway as the last couple of months, something I decided not to take into consideration when I originally posted this.