Conventional gasoline (CG) is the standard motor fuel sold at most gas stations across the United States. It’s a blend of over 150 hydrocarbon compounds refined from petroleum, mixed with additives and ethanol, and sold in the familiar octane grades you see at the pump: regular (87), midgrade (89–90), and premium (91–94). The term “conventional” exists primarily to distinguish it from reformulated gasoline (RFG), a cleaner-burning version required by federal law in areas with serious smog problems.
What’s Actually in It
Gasoline is not a single chemical. It’s a complex mixture of hydrocarbons, and finished blends can contain more than 150 separate compounds, with some analyses identifying as many as 1,000. The typical breakdown by volume includes 25–40% branched-chain hydrocarbons (isoalkanes), 20–50% aromatic compounds, 4–8% straight-chain hydrocarbons, 3–7% cyclic hydrocarbons, and smaller fractions of other carbon-hydrogen molecules. Benzene, a known carcinogen, makes up 0.5–2.5% of the total volume.
On top of the base hydrocarbon mixture, refiners add performance and stability additives. Nearly all conventional gasoline sold in the U.S. today contains about 10% ethanol (known as E10). This ethanol blend is the standard for most passenger vehicles, though cars from model year 2001 and newer can also use E15, which contains 10.5–15% ethanol. Older vehicles, motorcycles, boats, and small engines like lawn mowers are limited to E10.
Octane Grades at the Pump
The three grades of conventional gasoline you see at retail stations reflect different octane ratings, which measure a fuel’s resistance to knocking (premature ignition in the engine). Regular sits at 87 octane, midgrade at 89–90, and premium at 91–94. Higher-compression or turbocharged engines typically require premium to run properly, while most standard passenger cars are designed to run on regular. Using a higher octane than your engine requires offers no performance benefit.
How It Differs From Reformulated Gasoline
Reformulated gasoline is blended to produce significantly fewer volatile organic compounds (VOCs) and toxic air emissions compared to conventional gasoline. The biggest difference is a lower Reid Vapor Pressure (RVP), which is a measure of how easily the fuel evaporates. Lower vapor pressure means less gasoline evaporates into the atmosphere on hot days, and that RVP reduction accounts for the majority of RFG’s emission benefits.
That said, the gap between the two has narrowed over time. The EPA has harmonized several key standards so that both conventional and reformulated gasoline must now meet the same limits for benzene content and sulfur. Under Tier 3 regulations, all gasoline sold in the U.S. must contain no more than 10 parts per million of sulfur on an annual average, with an absolute cap of 80 ppm on any individual gallon.
Where Conventional Gasoline Is Sold
Conventional gasoline is the default fuel across most of the country. RFG is only required in specific metropolitan areas classified as having severe ozone pollution under the Clean Air Act. If you live outside those designated zones, you’re buying conventional gasoline.
The original RFG areas include parts of the Northeast, mid-Atlantic, and several large metro regions. More recently, the federal government has expanded the list. The Dallas-Fort Worth area in Texas and the Denver metro area in Colorado were reclassified as severe ozone nonattainment zones, and both transitioned to mandatory RFG sales in late 2023. Eastern Kern County in California made the switch in 2022. The entire state of California also operates under its own reformulated gasoline program. In all of these areas, selling conventional gasoline is prohibited by federal law.
Summer and Winter Blends
Conventional gasoline isn’t the same year-round. The EPA sets seasonal vapor pressure limits to reduce smog during warm months, when evaporating fuel contributes most to ground-level ozone. From June 1 through September 15, conventional gasoline must have a vapor pressure no higher than 9.0 psi. Some regions face a stricter limit of 7.8 psi. Gasoline containing 10% ethanol gets a 1.0 psi allowance on top of these caps because ethanol slightly raises vapor pressure.
Winter blends are allowed higher vapor pressure, which actually helps engines start more easily in cold weather. This seasonal switch is why gas prices often rise in spring: refineries need to produce the more expensive, lower-volatility summer formula.
Emissions and Environmental Impact
Every gallon of conventional gasoline burned produces about 8,887 grams of carbon dioxide. For a typical passenger vehicle, that adds up to roughly 4.6 metric tons of CO2 per year, or about 400 grams per mile driven. Beyond carbon dioxide, gasoline combustion releases methane and nitrous oxide from the tailpipe, both potent greenhouse gases. Vehicle air conditioning systems can also leak hydrofluorocarbons, adding another layer to the overall emissions profile.
Ground-level ozone, the main ingredient in smog, forms when VOCs and nitrogen oxides from gasoline evaporation and combustion react with sunlight. This is precisely why the EPA regulates vapor pressure so tightly in summer and mandates reformulated gasoline in the worst-affected metro areas. Conventional gasoline produces higher evaporative emissions than RFG, which is why it’s banned in severe nonattainment zones.