Twice a year, US gasoline quietly changes recipe: a low-evaporation summer blend for smog season (June 1–September 15), and a cheaper, butane-rich winter blend for cold starts. The switch drives the dependable spring price bump — and part of the autumn relief.
| Summer blend | Winter blend | |
|---|---|---|
| Vapor pressure (RVP) | ≈7.8–9.0 psi (low evaporation) | up to ≈13.5–15 psi |
| Purpose | Prevent smog-forming evaporation in heat | Reliable cold starts |
| Key ingredient change | Butane largely removed | Cheap butane blended in |
| Cost to make | ≈10–20¢/gal more | Cheaper |
| Required | June 1 – Sept 15 at retail (longer in CA) | Rest of the year |
| Fuel economy | Baseline | ≈1–3% lower energy content |
Metro areas with smog problems (and all of California) use even stricter blends — one reason for the regional price gaps on gas prices by state. The seasonal pattern shows up clearly in annual price history and in RBOB futures.
Vapor pressure. Summer gasoline is blended to evaporate less (lower RVP, ~7.8–9.0 psi) so it doesn't form smog in heat; winter gasoline evaporates more easily (up to ~13.5–15 psi, using cheap butane) so cold engines start. Same octane, different recipe.
Two reasons: summer blend can't use cheap butane filler, making it genuinely costlier to produce (typically 10–20¢/gal), and the springtime switchover coincides with refinery maintenance season and rising driving demand — the reliable April–May price bump. Winter blend's return in mid-September usually helps prices ease.
Refiners must ship summer-spec gasoline from May 1 and stations must sell it June 1 through September 15 (federal rule; California runs longer). Winter blend phases back in from mid-September — one reason fall gasoline is usually the year's cheapest.
Slightly — about 1–3%. Winter blend's butane content carries a little less energy per gallon, and cold weather itself (dense air, longer warm-ups, winter tires) costs several percent more. Most of the "winter MPG drop" people notice is the weather, not the fuel.