[pcmaher]

Lower octane fuel can cause an engine to detonate (knock) sooner. This is a bad thing for engines. Many modern cars have knock sensors that will cause the onboard computers to retard engine timing to reduce/eliminate knocking. The problem is that this also reduces power. If you use higher octane fuel, the engine will not have to retard the timing, giving you more power. I did a little experiment a while back using 91 octane vs. 93 octane (I don't know if octatne is measured the same over here in the States, doesn't sound like it). I got less mpg out of the cheaper gas, so using the cheaper gas did not really save me anything. I think the difference in cost turned out to be $1.50 per tankful. I decided to keep using the good stuff. The engine runs better, and the better gas probably deposits less crud.