Source: U.S. Energy Information Administration, based on the Environmental Protection Agency.

Note: Click map to enlarge.

Note: Map includes some partial counties and shows both required and Opt-In RFG areas. In California all counties implement a version of the California Reformulated Gasoline Program, not just those required by the Clean Air Act.

Note: Volatility is a measure of how easily a liquid (or solid) will change into a vapor. For gasoline, it is measured by Reid Vapor Pressure (RVP). The higher the RVP, the more volatile the gasoline. U.S. Energy Information Administration, based on the Environmental Protection Agency.Map includes some partial counties and shows both required and Opt-In RFG areas. In California all counties implement a version of the California Reformulated Gasoline Program, not just those required by the Clean Air Act.Volatility is a measure of how easily a liquid (or solid) will change into a vapor. For gasoline, it is measured by Reid Vapor Pressure (RVP). The higher the RVP, the more volatile the gasoline. RFG (reformulated gasoline) must meet even stricter limits on volatility.

Republished April 29, 11:30 a.m. to correct map.

Refiners are currently switching to make summer-grade gasoline ahead of the May 1 compliance date for refiners and product terminals. Summer-grade gasoline has a lower volatility than winter-grade gasoline to limit evaporative emissions that normally increase with warm weather and cause unhealthy ground-level ozone.

Volatility is a measure of how easily a liquid (or solid) will change into a vapor. For gasoline, it is measured by Reid Vapor Pressure (RVP). The higher the RVP, the more volatile the gasoline. While the Environmental Protection Agency (EPA) requires lower-volatility summer gasoline, aside from any government regulation, gasoline's RVP must be limited to ensure that the fuel does not vaporize in the fuel system. If it does, the engine can stop running.

It costs refiners several cents per gallon more to make summer-grade gasoline, compared with winter-grade fuel, which is part of the reason that retail pump prices can rise in the summer.

Additionally, in parts of the country that require cleaner, reformulated gasoline (RFG), such gasoline must meet even stricter limits on volatility (see map). The EPA, for example, requires use of RFG in high-smog areas to reduce smog-forming particulates and pollutants. States or regions also have gasoline quality requirements; California, for example, has stricter requirements than the federal government.

For logistical reasons, the transition to low-RVP gasoline happens over the course of several spring months as temperatures rise and to facilitate lowering the RVP of remaining inventories of winter-grade gasoline. The federally mandated dates for summer-grade gasoline and reformulated gasoline, where required, are May 1 to September 15 for refiners and terminals, and June 1 to September 15 for gasoline retailers. In California switchover dates are earlier, and summer-grade gasoline must be in use for a longer period.

A typical summer-grade gasoline is composed of fluid catalytic cracker gasoline, 40%; straight-run gasoline (directly from crude oil distillation), 25%; alkylate, 15%; reformate, 18%; and butane, 2%. Winter-grade gasoline usually contains more butane, which has an octane rating slightly below premium gasoline (91-93 octane). While butane is an economic component of gasoline, the high volatility of butane limits the amount of butane that can be used in summer-grade gasoline.