That depends entirely on the car, though. Our old ECUs have a max timing map, and a small amount of adjustment per cell. They won't gain any benefit from anything above 89 (and possibly not from that either). Newer cars are different and can dynamically adjust timing to account for fuel quality.in my old 300m, i used 93 and the extra mileage washed the cost. but the 93 bumped my long term timing maps up and anytime id put 87 in it, the car would run terrible the next 100 miles until it relearned. running higher octane gas will let your car advance timing further and pick up some performance and mileage to an extent. most people argue this because they think the results are instant, they arent, the ecu takes time to readjust its maps.