Gas

If you know what engine size you have type in the compression ratio for that size engine on Google, if it's under 9:1 than putting anything over regular gas is a waist of money, premium gas is designed for engines that have a higher compression than 9:1. If you have under 9:1 and your running premium gas your not ganging any benefits your just wasting gas literally it won't burn it all and just force it out of the exhaust and create more deposits in your engine

Use whatever your owner's manual says you should use. GM spend a bazillion dollars figuring all that out and programming your computer run the engine at peak efficiency with a certain . You're not going to outsmart the GM engineers by changing fuel and you may in fact make it run worse because the different octane level could make it operate at a different temperature which the computer will interpret as a problem of some sort and start compensating for it or showing trouble codes.

Your best bet would a cat back exhaust super easy to put on you only need a socket set and a pair of channel lock pliers for removing rubber exhaust hangers...I drive a 2010 cobalt it and it took less than 10 minutes including removal and the sound from new system is great not raspy but deep tone

i have an 08 cobalt chevy and i had someone put gas in for me . they put 87 instead of 89 which is what i usually get . now my car won’t start ?

I'm sure you have this resolved by now, but for what its worth the no start has nothing to do with the different octane rated fuel.You have another problem. Start by scanning it for trouble codes.