Should i use premium gas or regular in a 1999 jeep grand Cherokee?

The 1999 Jeep Grand Cherokee owner's manual will specify the recommended fuel octane rating. Generally, unless your specific model's engine requires premium (this would be clearly stated in the manual), regular unleaded gasoline is acceptable. Using premium in a car that doesn't need it won't hurt the engine, but it's a waste of money. Check your manual first – that's the definitive answer.