The cars today have a computor and knock sensors. They will detect knock and cut timing. So if you want the HP you have bought the car for, I would go with the advertised octane. If you use lower rated fuel you will lower the HP rating. I always go with 93 oct. and want the best HP I can get at all times. I even add 109 octane when going to the track so I get the best performance out of the engine I can get and to add insurance to keep the engine safe from detination. I would only use lower octane in the case of an emegency and I would also baby the car while the cheap stuff is in there.