Trooper 99: Please check your owner's manual for octane requirement. Typically, if an engine is running properly, it should not knock on the recommended gasoline. However, there are some exceptions: 1. Bad gas; sometimes (witness the problems in Atlanta with lead in unleaded gas) a gas station will get a bad load or, even worse, will sell bad quality gas purposefully. 2. Age of the engine; it used to be that octane requirement rose as an engine accumulated miles, usually due to carbon build-up in the combustion chamber. I don't know whether this is still a huge issue because of the improvements in detergent additives. 3. Changes in formulation; in some parts of the country, the gas is reformulated for the weather (i.e. winter blends, summer blends etc.) 4. Super heavy loads and really hot weather; this is can cause an engine to run hotter and low octane fuel can occasionally cause knock. Hope this helps.