I just copied this from the net - so might even be accurate 
The European octane rating is RON, whereas the US octane rating is an average of (RON+MON)/2. [RON +MON divided by 2]
EURO RON US (R+M)/2
90 = 85.9
92 = 87.8
95 = 90.7
96 = 91.6
98 = 93.5
100 = 95.5
105 = 100.2
110 = 105
The European octane rating is RON, whereas the US octane rating is an average of (RON+MON)/2. [RON +MON divided by 2]
EURO RON US (R+M)/2
90 = 85.9
92 = 87.8
95 = 90.7
96 = 91.6
98 = 93.5
100 = 95.5
105 = 100.2
110 = 105