I've always heard the money is in the stores and that the margins on the actual gasoline/diesel are razor thin. Granted this was a few years ago, so things could have changed.
I have one small store in the family and this is pretty much the case. You're only going to make a few cents on the gallon. I know you're far away from this point, but consider giving a discount on gas for paying cash just to get people inside the store.