I think it depends on the car and compression ratio. I am you RLSH700, I would be worried about pre detonation. Mostly on cars with forced induction, is when I would have say it would get worse mpg and possibly blowing up the engine.

When I had my grand prix gtp, it would knock so bad if I used 87, sometimes I would see up to 7 degrees of KR (knock retard). Which eventually caused the car to run supper lean and blew a chunk off of my piston.

I don't think if a vehicled call for regular and you put in high grade it would make a difference on mpg.