Well I normally use 87 Octane and get 22-26 mpg with my '95 SE Auto.
I have used 93 when it was around $1.30 a gallon and noticed a slight increase in mpg - can't say that I felt any difference.
If the car has a problem with lower Octane fuel you will soon know about it due to pinking or knocking of the engine. I have never experienced this to date - in fact I've never even observed this since I was a kid in one of my dads cars driving up a very steep slope many many years ago
