Summer vs Winter Gasoline
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.
The Two Blends
| 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.
RBOB Gasoline Futures by Month, Last 12 Months
| Month | Average, $/gal | High | Low | Month-end close | Blend in force at the pump |
|---|---|---|---|---|---|
| Oct 2026 | 3.262 | 3.403 | 3.115 | 3.115 | Winter |
| Sep 2026 | 3.362 | 3.587 | 3.104 | 3.438 | Summer until Sept 15, then winter |
| Aug 2026 | 3.186 | 3.490 | 2.839 | 3.437 | Summer (June 1 to Sept 15) |
| Jul 2026 | 3.227 | 3.496 | 2.917 | 3.222 | Summer (June 1 to Sept 15) |
| Jun 2026 | 3.020 | 3.144 | 2.881 | 3.014 | Summer (June 1 to Sept 15) |
| May 2026 | 3.503 | 3.761 | 3.127 | 3.127 | Winter at the pump; refiners ship summer grade from May 1 |
| Apr 2026 | 3.267 | 3.772 | 3.001 | 3.772 | Winter |
| Mar 2026 | 2.946 | 3.352 | 2.371 | 3.312 | Winter |
| Feb 2026 | 1.963 | 2.078 | 1.851 | 2.078 | Winter |
| Jan 2026 | 1.807 | 1.923 | 1.694 | 1.923 | Winter |
| Dec 2025 | 1.757 | 1.869 | 1.681 | 1.705 | Winter |
| Nov 2025 | 1.938 | 2.012 | 1.857 | 1.896 | Winter |
Calendar-month statistics of the front-month NYMEX RBOB futures, from this site's two-year daily history. The futures price also moves with crude and with the refining margin, so a two-year window cannot isolate the blend effect from them; use it to see how the month-to-month path looks, not to measure the 10–20 cents the summer blend adds. Seasonal averages of RBOB returns back to 2000: the prices page. Live price: RBOB gasoline.
Seasonal Gas FAQ
What is the difference between summer and winter gasoline?
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.
Why is summer gas more expensive?
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.
When does gas switch between summer and winter blend?
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.
Does winter gas give worse mileage?
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.