Why is winter gas cheaper, and when does it switch?
Winter gasoline is cheaper to make because the federal summer limit on Reid vapor pressure (RVP) generally stops applying after September 15, and a higher RVP lets refiners use more butane. EIA says it costs refiners several cents per gallon more to make summer-grade gasoline than winter-grade.
RVP measures how easily gasoline turns to vapor. EPA caps it in summer to cut smog-forming evaporation. In cold weather, EIA notes, higher volatility helps engines start more easily, and the federal cap applies only in the summer ozone season.
This covers gasoline only, from the supplier side: the dates that apply to you, what changes in the blend, and what to do with summer-grade product still in your tanks.
What dates does the summer volatility control period run?
For refiners, importers and terminals the federal summer season generally runs May 1 to September 15. For retailers and wholesale purchaser-consumers it generally runs June 1 to September 15. Those dates are EPA’s summary of 40 CFR 1090.215, and EIA gives the same split.
| Who | Summer season (generally) | Ends |
|---|---|---|
| Refiners, importers, terminals | May 1 | September 15 |
| Retailers, wholesale purchaser-consumers | June 1 | September 15 |
The staggered start gives product time to work through the system. EIA says the move to low-RVP gasoline happens over several spring months to lower the RVP of remaining winter-grade inventory. Under the federal rule both groups end on the same day, September 15, unless an approved state plan sets a longer control period. After that date the federal summer cap no longer applies; separately, 40 CFR 1090.215(d)(1) exempts gasoline designated as winter gasoline that was not delivered to a retail outlet or WPC during the summer season.
Why does higher RVP mean more butane?
Butane is volatile, so it adds a lot of RVP, and the summer cap squeezes how much of it fits. EIA describes butane as a low-cost octane booster whose high RVP limits its use in summer and RFG blends. Low-RVP gasoline uses costlier components such as alkylate instead.
EIA’s older changeover note gives a typical summer-grade blend with butane at 2%, and says winter-grade gasoline usually contains more butane. That is the mechanism: a looser RVP limit lets in more of a cheap component, so it costs less to make. EIA does not give a single current cents-per-gallon figure for the gap.
Does the same rule apply everywhere?
No. The federal ceiling is 9.0 psi in summer, but lower limits apply in some places. Per EPA and 40 CFR 1090.215:
| Variant | Summer limit | Where |
|---|---|---|
| Federal standard | 9.0 psi | Default for the U.S. |
| Federal 7.8 psi | 7.8 psi | Areas listed in the regulation |
| Reformulated gasoline (RFG) | 7.4 psi | RFG covered areas |
| SIP-controlled | Set by the state plan | Areas with an approved state fuel rule |
Some details change what you can ship:
- Gasoline subject to the 9.0 or 7.8 psi standard generally gets a 1.0 psi allowance when it meets the ethanol requirements, but not everywhere. EPA lists states such as Maine, New York, Texas and Vermont where state rules do not allow it, and states that have opted out.
- Gasoline in an area with more than one standard must meet the most stringent one.
- In 7.8 psi areas, 9.0 psi summer gasoline may be located there between May 1 and May 31.
- California has its own rules. EIA says its summer limit is 7.0 psi and its summer-grade period is longer.
- Alaska, Hawaii and U.S. territories are exempt from the federal volatility rules.
Some state programs set their own dates, and EPA says affected parties should check the state plan. If you serve more than one state, your changeover date is a per-state question.
What happens to summer-grade inventory in October?
The federal limit is a maximum, so summer-grade product left in your tanks is not out of spec just because September 15 has passed. The job is working it through your system before you load the next grade. That is a sell-down plan, not a regulatory deadline, and the numbers below are illustrative.
Say a terminal tank holds 120,000 gallons of summer-grade gasoline and the site moves 30,000 gallons a week.
| Week | Start of week (gal) | Sold (gal) | End of week (gal) |
|---|---|---|---|
| 1 | 120,000 | 30,000 | 90,000 |
| 2 | 90,000 | 30,000 | 60,000 |
| 3 | 60,000 | 30,000 | 30,000 |
| 4 | 30,000 | 30,000 | 0 |
At that pace the tank is clear in four weeks, which is just inventory divided by throughput. A slower site takes longer. If you refill during those weeks, the new product mixes with the old, so you can stop topping up summer grade and let the tank draw down first. Whether to mix or wait is your call and your terminal’s; EPA and EIA do not prescribe it.
Two practical checks while the changeover runs. First, confirm the product description on the delivery ticket and bill of lading matches what actually loaded. Second, if your drivers load at a carded terminal, expect the rack to move to winter grade on its own schedule, so ask which grade a load is before it is booked against a customer’s order.
How does the changeover show up in rack prices?
Rack prices move, and the cheaper winter blend is one reason they can. EIA frames the cost difference as one reason pump prices rise in summer. It does not say the winter drop is automatic or a fixed size, and other drivers move the rack at the same time, so do not quote a customer a fixed seasonal saving.
How a rack price gets set is a separate topic. For the buyer’s view of comparing delivered offers, see how to compare bulk fuel quotes. For the cost of getting product from the rack to the tank, see fuel hauling rates, per mile to per gallon. The guide to wholesale fuel pricing for suppliers covers the price side in more depth.
How Energy Connect handles it
When you switch grades, and what you charge for each, stays your decision and your terminal’s.