Recently there was a thread on the subject of whether or not premium gas will give better mileage. I would have added this to it but could not locate it again.

Having seen the original post, I decided to try it and see what the results were. I have an 88 745 GLE with M46 and 3.31 rear, with 115K on it, in very good mechanical condition. It normally gets 29-32 MPG on 87 octane gas. I tried a tank of 93 and found no difference whatsoever in mileage (yes, I know it's hard to tell for sure on only one tank, but I filled up at the same pump both times, and the automatic shutoff is pretty consistent there) nor did I notice any major improvement in performance. It might have revved out a little smoother on the premium, but as I don't often feel the need to take it over 3000 rpm, that is hardly worth the difference in price. It didn't seem to have any additional low speed torque either.

Conclusion: in my car, premium gas is a waste of money. This is not to say that no car will benefit from it, only that mine did not seem to.