Originally Posted by
jdb840
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.
From what I have observed and especially in the case for forced air induction, what you say is true. My friend had a 98 GP GTP and only once did he put regular in it. After that, he stuck with premium and the highest octane premium he could find. I think that in a case for using higher than regular grade in a regular grade requiring car, where you have flat roads with no hills and the risk of downshifting does not exist, there is probably little to no difference, but in the situation that I observed, I believe a difference would arise.