The short answer: three things at once
Every spring, the national average climbs toward summer, and every fall it eases back. In a normal year, gas in August has run about 40 cents a gallon higher than gas in January. Three things cause that, and they all land around the same time:
- A cleaner, costlier "summer blend" is required by law from June through mid-September.
- Everyone drives more in summer, so demand peaks.
- Refineries do yearly maintenance in early spring and switch their recipe over right as driving season starts, which tightens supply.
Here is the part most people get wrong: the summer blend itself is the smallest of the three. It adds only a few cents. The other two do most of the work. Let me walk through each.
What "summer blend" actually is
Gasoline evaporates. How easily it evaporates is measured by something called Reid Vapor Pressure, or RVP, and that is the whole story behind summer blend. In hot weather, gas that evaporates easily puts more fumes into the air, and those fumes help form smog. So the EPA requires a less-evaporative blend in summer, with a lower RVP, to cut down on those emissions when the heat is worst.
In winter it flips. A blend that evaporates more easily helps your engine start in the cold, so the rules loosen. It is the same gas pump and the same octane, just a different recipe depending on the season. The summer recipe runs from June 1 to September 15 at the pump.
Why the blend costs a little more to make
To make gas evaporate less, refiners have to pull out a cheap, common ingredient called butane and replace it with pricier components that hold the octane up while bringing the evaporation down. That extra step costs money. But not much money. The Energy Information Administration says the summer recipe costs refiners only "several cents per gallon" more than winter gas. An industry group puts the high end at around 15 cents.
So if the blend only adds a few cents, why does summer gas often cost 40 cents or more above winter gas? Because the recipe is not working alone.
The bigger reasons: demand and the spring crunch
The two heavier weights on summer prices have nothing to do with the recipe.
First, demand. People drive more in summer, road trips, vacations, longer daylight, so the country burns more gas right when supply is tightest. More buyers chasing the same fuel pushes prices up.
Second, the spring supply crunch. Refineries schedule their big yearly maintenance for the first quarter, after winter heating demand fades and before summer driving ramps up. While they are down for maintenance, and while they are switching their equipment over to make the summer blend, they produce less gas, and inventories get drawn down. That happens at the exact moment demand is rising. The EIA finds the average spring run-up lasts about 16 weeks, usually starting in early February and topping out around Memorial Day.
Add it up and the picture is clear: the roughly 40-cent jump from winter to summer is the blend plus the demand plus the spring crunch, all together. The blend is the small slice everyone names because it has a catchy label. For more on why pump prices lag the oil market by weeks, see why gas prices rise faster than they fall.
Why California is always the most expensive
Some regions require an even stricter, special blend called reformulated gas, mandated by the Clean Air Act in the smoggiest areas (about a quarter of US gas sales, concentrated in the Northeast, around Chicago and Milwaukee, and Southern California). California goes furthest of all, with its own unique state recipe that evaporates even less than the federal summer blend.
That is the quiet reason California leads the country almost every summer. Most refineries outside the state are not built to make California's blend, so when California supply gets tight, it cannot easily be topped up from elsewhere. That isolation, sitting on top of the state's higher gas taxes and fees, keeps California pumps the priciest. We go deeper in why California gas prices are the highest in the US.
When it eases up
The rhythm is fairly predictable. Prices climb from around early February through late spring, hold through the peak summer driving months, and then ease in the fall. Two things flip in autumn: after September 15 stations can sell the cheaper winter blend again, and people drive less once summer ends. Both pull prices down. A big swing in crude oil can override the seasonal pattern in any given year, but the underlying shape shows up almost every time.
What you can actually do about it
You cannot shop your way around the summer blend. From June through September it is simply what is in every pump in the country, so there is no formulation to choose. What you can control is which station you fill at. Within a single metro area, the cheapest and most expensive stations are routinely 40 to 80 cents a gallon apart on the same day, which is more than the entire summer blend premium. So the move that actually saves you money is checking before you fill up. Search prices by ZIP code for where you are, and for the best day and time to fill, see when to buy gas.
The other lever is making each gallon go further, which matters most in summer when the AC is running and the heat is hard on your car. Keeping tires properly inflated and staying current on basic maintenance both nudge your mileage up:
And since summer is peak road-trip season, the long highway miles are also when roadside coverage and a quick insurance comparison tend to pay off:
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Bottom line
Summer gas costs more because three things pile up at once: a cleaner blend that costs a little extra to make, a wave of summer driving, and the spring refinery slowdown that tightens supply. The summer blend gets all the blame, but it is the smallest piece. None of it is something you can opt out of, but the gap between stations near you is bigger than the seasonal premium, so the real savings come from filling at the right pump, not from waiting for fall.