Nothing!!!! It wont do anything to performance!! Opti is only good for its cleaning properties. Its a bloody great con, its doesnt r****d it enough to effect anything, were only talking at 2 RON if the fuels brand new which chances are it aint so its more then likely 1RON if that! Ive done my own, admitable not very scientific tests, qtr miled at Pod on normal fuel, ran 15.07, few weeks later after a few tanks of 97 ron went to York and ran 15.04, feck all difference. Evo mag did some proper tests ages back on 4/5 of there fast fleet, none of the cars noticed anything more then .2 of a second difference performance wise, most nowt at all, only difference was the valves were cleaner after using Opti. Sure they had a BMW M Coupe, valves were clean before and after, performance difference was nothing, think it was .02 quicker to 100 or summit daft like that. Now if Opti makes that much difference to a M Coupe, then is it really gonna make much difference to a 172??!