Reformulated gasoline (RFG) is a cleaner-burning type of gasoline specifically blended to produce fewer smog-forming and toxic emissions than conventional gas. Congress mandated the reformulated gasoline program through the 1990 Clean Air Act amendments, requiring its use in parts of the country with the worst ground-level ozone pollution. Today, roughly 30% of gasoline sold in the United States is reformulated.
How RFG Differs From Conventional Gasoline
The core difference is chemical composition. Reformulated gasoline must contain a minimum of 2% oxygen by weight, achieved by blending in oxygenates that help the fuel combust more completely. It also caps benzene, a known carcinogen, at a maximum of 1% of the fuel’s volume. California’s version goes further, setting limits on aromatic hydrocarbons and certain distillation temperatures, which together lower the fuel’s overall density and reactivity.
In practice, the oxygenate used in virtually all modern RFG is ethanol, blended at about 10% by volume. That 10% ethanol translates to roughly 3.5% oxygen by weight, exceeding the federal minimum. Earlier formulations relied on a chemical called MTBE, which was effective at reducing air pollution but contaminated groundwater supplies and was phased out in most states by the mid-2000s.
Where RFG Is Required
The EPA designates ozone nonattainment areas, cities and metro regions that fail to meet federal air quality standards, as mandatory RFG zones. Beyond those mandated areas, state governors can voluntarily opt their regions into the program. As of the most recent federal count, roughly 43 to 46 areas across the country are covered. Major metro areas like Los Angeles, Chicago, New York City, Houston, and Philadelphia all require RFG year-round. If you live outside these zones, you’re buying conventional gasoline, which is any finished motor gasoline that doesn’t have to meet RFG standards.
Emission Reductions
The pollution benefits are substantial. A study from Lawrence Berkeley National Laboratory measured tailpipe emissions in California between 1994 and 1997, the period when the state transitioned to reformulated fuel. Carbon monoxide dropped by 31%, smog-forming volatile organic compounds (VOCs) fell by 43%, and nitrogen oxides decreased by 18%. The effect was consistently stronger for VOCs than for nitrogen oxides, which makes sense given that RFG primarily targets evaporative and combustion-related hydrocarbon emissions.
RFG also cuts toxic air pollutants. Benzene emissions drop by about 33% compared to older conventional blends, and 1,3-butadiene, another carcinogen released from vehicle exhaust, falls by over 40% in some comparisons. Formaldehyde and acetaldehyde see more modest reductions, typically in the range of 2% to 13% depending on the baseline and the specific oxygenate used.
What It Costs You at the Pump
RFG consistently costs more than conventional gasoline. Recent EIA retail price data shows reformulated areas averaging around $3.25 to $3.47 per gallon, while conventional areas sit closer to $2.84 to $2.98 per gallon. That works out to a premium of roughly 40 to 50 cents per gallon, though the gap fluctuates with refining costs, regional supply constraints, and seasonal blending requirements. Part of the premium reflects the more complex refining process, and part reflects the fact that RFG areas tend to be large metro regions where gas prices run higher for other reasons too, like higher taxes and real estate costs for stations.
Fuel Economy Impact
Because ethanol contains less energy per gallon than pure gasoline, RFG delivers slightly fewer miles per gallon. The EPA estimates a 1% to 3% reduction depending on the season. Summer RFG contains about 1% less energy than summer conventional gas, so you lose roughly 1% fuel economy. Winter RFG, which can have a different blend profile, contains about 3% less energy per gallon, and a Wisconsin field study confirmed a 2.8% fuel economy drop during winter months. For a car that normally gets 30 MPG, that translates to losing somewhere between a third of a mile and about one mile per gallon. Most drivers won’t notice the difference without careful tracking.
Summer and Winter Blending Requirements
Gasoline volatility, how easily it evaporates, is a major factor in smog formation. The EPA regulates this through a measurement called Reid Vapor Pressure (RVP) during the summer ozone season, which runs from June 1 through September 15. Standard gasoline can’t exceed 9.0 psi during this window, but many RFG areas face a stricter limit of 7.8 psi. Starting in 2024, the Denver area was brought under an even tighter standard of 7.4 psi.
Lower volatility in summer means less gasoline evaporates from fuel tanks, pumps, and engine systems on hot days, cutting the hydrocarbon vapors that cook into smog under sunlight. Winter blends are allowed higher volatility because cold engines need more vapor to start reliably, and ozone formation is less of a concern in cooler temperatures. This seasonal switchover is one reason gas prices often rise in spring: refineries have to retool production for the more tightly controlled summer blend.
Compatibility With Vehicles and Equipment
For modern cars and trucks, RFG causes no compatibility problems. Vehicles built after the early 1980s have closed-loop fuel systems that automatically adjust the air-fuel mixture to account for the extra oxygen in reformulated blends. Older cars, particularly those made before 1981, run open-loop systems that can’t compensate, causing the engine to run slightly leaner. In very old vehicles from the mid-1970s, RFG’s solvent properties can dissolve lacquer deposits in fuel lines and clog filters, though this is a one-time issue that resolves after a filter change.
Small engines like those in lawnmowers, chainsaws, and leaf blowers are generally compatible. Manufacturers have reported no performance or materials problems within standard ethanol blend limits. The one concern is with two-stroke engines: if water enters the fuel system, ethanol-blended gasoline can separate into layers, leaving the engine running on a lean, unlubricated mixture. Proper fuel storage and keeping water out of gas cans prevents this. Marine engine manufacturers have similarly confirmed that ethanol blends up to 10% don’t pose significant problems for boat engines.