Explain how use of EGR is effective in reducing NOx emissions 4. In most locations throughout the U.S., the octane number of regular grade gasoline [defined as (R+M)/2] is 87. However, in certain high elevation locations (parts of CO, ID, MT, UT, and WY) regular gasoline may have a lower octane value of 85. Explain how a lower octane fuel can be used in these areas without it leading to unacceptable knocking