Octane is the US is the average of MON and RON as stated in the other reply, which usually places it about 5 point below the RON number which is used in Europe

Octane is the US is the average of MON and RON as stated in the other reply, which usually places it about 5 point below the RON number which is used in Europe and many other places. Also, in higher altitudes you don't need as high an octane, so where I live (4000ft-9000ft) premium is 91.