U.S. Gasoline Types: Conventional, Reformulated, Regional, and Seasonal Blends, Octane Grades, Reid Vapor Pressure Limits, and State Differences

     

      The Energy Information Administration (EIA) has long done a good job explaining the details of gasoline types and differences. They explain that in the U.S., motor gasoline is a blend of different hydrocarbons and chemicals. The exact blends vary by season and by region. The EPA requires different blends in regions with air quality concerns.

“Key differences between formulations include octane rating, volatility—commonly measured as Reid vapor pressure (RVP)—and emissions. “This year, the EPA will relax federal enforcement of summer RVP standards to help reduce gasoline prices.”

     There are two main types of American gasoline formulations: conventional and reformulated (RFG). Most of the U.S. uses conventional gasoline, which meets air quality standards in those areas. RFG is required where conventional gasoline would exceed smog air pollution limits in accordance with the Clean Air Act. RFG burns cleaner but costs more to produce. The EPA notes that about 25% of U.S. gasoline is RFG. The first phase of the RFG program began in 1995, and the second (current) phase began in 2000. RFG is currently used in 17 states and in the District of Columbia. California requires a blend called Phase 3 RFG. RFG requirements have helped to reduce summer smog in vulnerable cities. The EPA explains some other important aspects of the RFG program below:

“Building on the success of the RFG program, EPA has harmonized requirements that apply to both conventional gasoline (the typical gasoline used in non-RFG areas) and RFG. For example, CG and RFG must both comply with same standards for benzene and sulfur content. This ensures that both CG and RFG provide similar substantial reductions in emissions of toxic pollutants and emissions of nitrogen oxides, which react in the atmosphere to form smog. In addition, RFG continues to provide reductions in emissions of volatile organic compounds (VOCs) from onroad and nonroad sources during the summer due to its lower vapor pressure, which is a measure of how easily gasoline evaporates. VOC emissions also contribute to the formation of smog.”

 

Octane Ratings

     Both kinds of gasoline are available in different grades, the familiar regular, midgrade, and premium varieties, which vary by octane. Ethanol may be blended with both types.

“The octane rating is the measure of fuel stability. The rating is based on the pressure at which a fuel spontaneously combusts (auto-ignite) in a testing engine. The octane number is the simple average of two different octane rating methods—motor octane rating (MOR) and research octane rating (RON)—that differ primarily in the specifics of the operating conditions. The higher an octane number, the more stable the fuel.”

     The familiar grades and their octane numbers are shown below.

     Octane is C8H18. It has 18 isomers, or different molecular configurations.

“Of the 18 isomers of normal octane (C8H18), octane gets its name from the 2,2,4-Trimethylpentane compound, which is highly resistant to auto-ignition. This iso-octane has been assigned the reference value of 100 for testing purposes. The extremely unstable normal heptane (C7H16) molecule is the 0 octane reference fuel.”

     Below, they explain how octane levels affect how the engine runs and burns fuel, potentially producing knocking and pre-ignition.

     Modern engine computers can minimize knocking and pre-ignition by reacting to them quickly when they occur. Octane is measured with an octane testing engine.

 

Seasonal and Regional Blends Vary by Reid Vapor Pressure (RVP) Limits

     Seasonal changes to gasoline blends are mainly changes in volatility as measured by Reid Vapor Pressure (RVP).

“…the lower the RVP, the less volatile the gasoline and the less evaporative the emissions. To reduce smog-forming emissions, the EPA mandates that summer grade gasoline has a lower RVP (less volatility) to control evaporation, which would normally increase in warm weather. In cold weather, higher volatility helps engines start more easily.”

     The EPA regulates different regions according to different RVP limits. Below, the EIA explains how RVP limits change by region and through the year.

     Lowering RVP for summer blends requires the addition of more expensive components such as alkylate. In winter, it is OK to use less expensive butane to raise octane values, but it also raises RVP, so alkylate is used instead to raise octane values in summer. 

     Below is a map of the U.S. showing summer blends in different states and regions.

     State blends include State Implementation Plan (SIP), Arizona Cleaner Burning Gasoline (AZ-CBG), and California Reformulated Gasoline (CA RFG). California has a summer RVP limit of 7.0 psi and extends the summer blend time periods in different regions as shown below.

References:

 

What’s in your gasoline? Understanding U.S. motor gasoline formulations. Energy Information Administration. April 15, 2026. What’s in your gasoline? Understanding U.S. motor gasoline formulations - U.S. Energy Information Administration (EIA)

Gasoline explained. What is octane? Energy Information Administration. Gasoline explained - octane in depth - U.S. Energy Information Administration (EIA)

EPA Fortifies Domestic Fuel Supply, Provides Americans with Relief at the Pump by Approving Nationwide E15 and Removing Boutique Fuel Markets for E10. U.S. EPA. March 25, 2026. EPA Fortifies Domestic Fuel Supply, Provides Americans with Relief at the Pump by Approving Nationwide E15 and Removing Boutique Fuel Markets for E10 | US EPA

Reformulated Gasoline. U.S. EPA. Reformulated Gasoline | US EPA

Gasoline Reid Vapor Pressure. U.S. EPA. Gasoline Reid Vapor Pressure | US EPA

Alkylation is an important source for octane in gasoline. Energy Information Administration. February 13, 2013. Alkylation is an important source for octane in gasoline - U.S. Energy Information Administration (EIA)