You will affect the performance and potentially cause mechanical issues long term. When it starts affecting the engine, you will hear a little "knocking" sound. If it's just a short term issue (like waiting for the next payday--one or two fill-ups), then I wouldn't worry. If it's becoming a long term issue, does your local Sam's or Costco have a gas station in it? You can seriously recoup the membership price in gas alone if you're buying premium. Costco also has a AmEx Costco card that allows you to earn cash back, even on gas purchases.Good luck!