The US uses a weird octane rating system. It's the average of the Research Octane Number and the Motor Octane Number. Back in the day, the number on the pump was the RON. RON and MON are usually from 8 to 10 numbers different, so to approximate the US rating from a Research Octane Number you usually subtract 4 or 5. That puts the 98 RON down to 92 - 93 by the US.

Altitude plays a big role and the higher the altitude, the lower the cylinder pressure and lower the octane requirement. So Regular is 87 up to some altitude limit, maybe 5000 ft, then Regular will be 85 The other grades of gasoline follow.

R.