Can I use 85 grade gas if my car manual says to use 87 grade?

it is not recommended to use lower octane rating than the one specified by the Mfg. 85 octane burns hotter. this could cause your combustion chamber to get hotter than normal resulting in slightly overheating engine.

use fuel additives if u would be using 85 octane.lower octane fuel can damage the valves.usually thre will be carbon deposit on valves;in extreme cases the valves get bent.

also there would be some knocking in the engine.so always use additives.

How new is your car? If you don't use at least 87 grade then it might void your warranty.

better not. you can use higher octane rated gas but going below the safer limit is not advisable for the engine.

higher the octane, the better.

you will ok, unless, you hear a pinging sound. then switch up to a higher grade or use octane booster.

sounds like you're in a high altitude region like colorado or utah? you should be okay if that's the case, but if you are closer to sea level then no, you dont want to do that..

It won't make your car break, but your engine will not run as smoothly.

LOL - yes you can ... if it starts to "knock" switch back although it's highly unlikely ...