We're talking about (R+M)/2 ratings, not RON ratings.
91 RON is equivalent to 87 octane, 98 ron is equivalent to 94 octane (using the estimate from the link Roadworthy posted).
Another interesting quote from his link is, "Using a higher grade than is required will not usually increase performance. However, if the vehicle is equipped with knock sensors, as many late model vehicles are, a higher octane grade may enhance performance."
Hey it is an octain raiting not a RON ( sorry I thought it was the same thing, I'm by no means an expert in this stuff ) it may not always make a difference in fuel economy, but in some cars it defiantly dose. ( my old corolla went from struggling to get 600 Kms to the tank on 91 to 750 with 98 no worries with a lot more power, it also didn't ping off it's head )
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