i run 91 in my miata. there actaully is a noticeable difference when i run 89. it feels like im driving into a 35mph wind and my car bogs ALOT below 4g. i PROMISE you can feel it. ill let somebody borrow my car who doesnt beleive me. lol

in high mileage motors, carbon deposits can increase compression and create hotspots, making the fuel more likely to detonate and causes "ping".