Summer Gasoline is Here Again

We run a version of this post each year, since it is an issue that comes up each year. It is obviously not the only factor affecting gasoline prices. The recent decrease in oil prices from $85+ per barrel to around $72 per barrel will tend to act in the opposite direction. - Gail

Just what is summer gasoline? Twice a year, in the fall and in the spring, you hear about the seasonal gasoline transition. However, most people probably don’t understand what this actually means. AAA published a Top 10 list explaining the seasonal rise in gasoline prices, and summer gasoline checked in at #7:

7. The summer blend switchover. This transition from winter-blend to summer-blend fuel, a concoction that causes less smog, occurs every spring. It causes a dip in gasoline supplies as refineries in the U.S. shut down temporarily to retool their production facilities.

That's only partially correct, and is probably the extent of most people's understanding of this transition. But given that I am very keen that people should understand the energy industry, it is worth a review, and a layman's explanation. I explained the details behind this transition in Refining 101: Winter Gasoline. But let’s review some concepts.

There are two key (although not the only) specifications that refiners must meet for gasoline. The gasoline needs to have the proper octane, and it needs to have the proper Reid vapor pressure (RVP). While the octane of a particular grade is constant throughout the year, the RVP spec changes with the seasons.

The RVP is based on a test that measures vapor pressure of the gasoline blend at 100 degrees F. Normal atmospheric pressure varies, but is usually around 14.7 lbs per square inch (psi). Atmospheric pressure is caused by the weight of the air over our heads. If a liquid has a vapor pressure of greater than normal atmospheric pressure, that liquid boils. For example, when you heat a pan of water, the vapor pressure increases until it reaches atmospheric pressure. At that point, the water begins to boil.

In the summer, when temperatures can exceed 100 degrees F in many locations, it is important that the RVP of gasoline is well below 14.7. Otherwise, it can pressure up your gas tanks and gas cans, and it can boil in open containers. Gas that is vaporized ends up in the atmosphere, and contributes to air pollution. Therefore, the EPA has declared that summer gasoline blends may not exceed 7.8 psi in some locations, and 9.0 psi in others. The particulars vary, but key considerations are the altitude and motor vehicle density of a specific location. The EIA summarizes the key points:

As gasoline evaporates, volatile organic compounds (VOC’s) enter the atmosphere and contribute to ozone formation. Gasoline’s propensity to evaporate is measured by Reid vapor pressure (RVP). In order to control VOC emissions, the Federal Clean Air Act Amendments of 1990 require that all gasoline be limited to an RVP maximum of 9.0 psi during the summer high ozone season, which the Environmental Protection Agency (EPA) established as running from June 1 to September 15. The Act also authorized the EPA to set more stringent standards for nonattainment areas. As a result, EPA limits areas designated as “high volatility non-attainment” to a maximum RVP of 7.8 psi during the high ozone season. Some States elected to require even more stringent restrictions to achieve local clean air goals, and require 7.2- and 7.0-psi gasolines.

Butane, which has an RVP of 52 psi, can be blended into gasoline in higher proportions in the winter because the vapor pressure allowance is higher. There are two advantages in doing this. First, butane is a cheaper blending component than most of the other ingredients. That makes fall and winter gasoline cheaper to produce. But butane also adds to the total gasoline pool, so that means that gasoline supplies increase in the winter as more butane is thrown into the mix. Not only that, but this all takes place after summer driving season, when demand typically falls off. These factors normally combine each year to reduce gasoline prices in the fall (even in non-election years). The RVP is stepped back down to summer levels starting in the spring, and this usually causes prices to increase.

There are some common misconceptions about this seasonal transition. One is that it is the reason that spring and fall maintenance are done. That is not the case. Most, if not all refineries can carry out this transition without shutting down or interrupting production. The reason that maintenance is done in the spring and fall is that it provides a combination of moderate weather (the inside of a vessel can be unbearable in the summer) and off-peak demand. Vessels must be inspected, new equipment must be installed, catalyst change-outs occur, etc. This is similar to tuning up your car to keep it in proper running condition. But the seasonal maintenance is unrelated to the gasoline transition. In fact, for reasons I won't get into here, seasonal maintenance often complicates the transition.

Another misconception that some have is that they can save money by buying cheap gas in the winter and storing it for the summer. Remember that winter gasoline will pressure up as the weather heats up, and the contained butane will start to vaporize out of the mix. You will end up with less gasoline than you paid for, and you will be contributing to the air pollution problem that summer gasoline was designed to avoid. If, on the other hand, you were to buy summer gasoline and try to store it until winter, you might find yourself having problems getting the fuel to ignite, due to the lower vapor pressure. This would be like putting a little bit of diesel in your gasoline – not very good for your car. So buy and use gasoline in the correct season.

Transition Schedule

The EPA publishes a schedule for the RVP transition:

Guide on Federal and State Summer RVP Standards for Conventional Gasoline Only

The schedule varies somewhat from region to region, but in general is as follows. After allowing vapor pressures as high as 15 psi in the winter, the limit drops on May 1st:

May: 9.0 psi
June – Sept. 15: 7/7.8 psi

More congested areas and hotter areas will tend to have a limit of 7.0 psi, while cooler climates generally opt for 7.8 psi. Some cooler climates don't even require a reduction, and have a 9.0 psi limit throughout the summer.

Refiners will start to pull down their inventory of winter gasoline well in advance of the May 1st deadline. On that date, all gasoline in the system has to meet the stricter requirements. This is a key reason that gasoline starts to become more expensive in the spring.

One of the disadvantages of having different requirements for different areas is that summer gasoline is less fungible. This can cause price imbalances in different areas, and sometimes prevents product from flowing from one area into another to ease the shortage.


Hopefully that was an easy-to-understand explanation of the seasonal gasoline transition in the U.S. The purpose of the transition is to curb pollution as the weather turns warmer. Now the next time you hear "season gasoline transition", you will know exactly what they are talking about and what the expected impact on supply and price will be.

The RVP of pure ethanol oxygenate is 18.0 psi,
so in E15 gasohol would contribute 2.7 psi to the RVP of gasoline.
The RVP of methyl-tetra-butyl-ether(MTBE) oxygenate is 8.0 psi.
In order to use ethanol in summer gasoline(RVP of 7.0 psi) you need to remove more pentane from gasoline prior to blending than you do with MTBE.

Unfortunately, MTBE seriously contaminated groundwater and was removed from use by Bush.

Are we having a mars mission problem? The RVP of pure ethanol is 2.5 psi but 17 kPa.

To make things fun, mixing E10 with gasoline raises the RVP by 1.0 psi. That's because gasoline and ethanol don't like each other, not because ethanol is more volatle then gas. It could be that E15 raises the RVP by 2.7psi (I don't have that number), but it isn't because ethanol is a gas at 100F.


Where are you coming up with this stuff?

The US government uses 18 psi for ethanol.

The Reid vapor pressure of ethanol in gasoline blending is 18 psi versus an MTBE Reid vapor pressure of 8 psi.

Wikipedia says ethanol is 9 kPa at 20 degrees C(68 degrees F).

The vapor pressure of ethanol is 2.45psi at 102F. You don't have to be a chemist to know that drinking alcohol is a liquid at room temperature.

Where are you coming up with this stuff?

The other Robert explained it well, but you can't do a flat vapor pressure calculation to come up with ethanol's contribution to the RVP. This is because of ethanol's polar nature and the hydrocarbon nature of gasoline. When you mix the two, there is a tendency for ethanol to push away from the hydrocarbons, raising the vapor pressure when you would otherwise expect it to be lowered.

Thanks for a great explanation. Two questions:

1) What do they do with the useless butane and other high-vapor-pressure components they produce during the summer?

2) Does Hawaii follow the same rules?

They can use it in as fuel in the refinery, flare it off (worst case), sell it for use as cooking fuel (LPG is mostly butane), use as feedstocks for making chemicals used in plastics, butane for cigarette lighters; isobutane is commonly used as a propellent is aerosol cans, a refrigerant in place of CFCs and it also is used to make butyl (synthetic) rubber.

Refineries waste very litte, if there isn't a demand for something they can store it until there is or they can use it in a different way, or crack it down, reconfigure it, change it and make something different. Gasoline is hard enough to blend, but diesel can be worse and there are very tight laws around sulfur content and it too has winter and summer formulas plus formulas for on-road and off-road use.

1. Butane is stored in underground caverns and pressurized tanks, and the inventory is pulled down in the winter.

2. Hawaii has the same sort of octane and RVP rules, but given the year round nature of the weather here, I don't believe there is any difference in summer and winter gasoline.

The reason prices go up in the summer is because they can. It is a conspiracy. Typically conspiracies are false, and are just a result of paranoia or ignorance. When conspiracies are proven to have actually happened, they are almost greed driven. Normally, it is just considered a drive for profits that all good capitalist try to achieve. It is when prices are illegally manipulated that makes a real conspiracy. Personally, I never used to believe in any conspiracies, until it was proven that the California blackouts were actually a conspiracy of greed. I also remember milk price fixing when I went to elementary school. Again, a proven conspiracy of greed.
Yes, I do believe that government regulation increases costs for 'summer' gas, but then why do the oil companies and refiners not claim a winter discount? Why does gas always cost 9/10 of cent as the last digit? Because the consumers will accept it that way.

Conspiracy? I don't think so. Try pricing butane versus the other chemicals that can reduce RVP. In addition, the summer gas can't be blended with winter gas so tanks have to be completely drawn down and pipelines empty. So there is a restocking cost, but it's not huge. Different chemicals do change the pricing, can't say how much. But there is difference. Around here prices haven't gone up or down a lot recently but where I am is one of the Metro areas that has to have "cleaner" gasoline all year.

Price does go up because it can, but that is because the demand curve for gasoline is almost verticsl (i.e. inelastic). People have got to drive to work and other places so they but gas and maybe they don't buy something else. If they cut back on the gas habit maybe price would have an impact on demand and thus more price elasticity would be present.

Now if you want to talk conspiracy why does a $10bbl change DOWNWARD in the price of oil never show up at the pump EVER? At 42 gallons per bbl and really no change in refining or distribution costs we should a cost decrease of about 20 cents, right? Maybe it's the same answer, because they CAN, because we keep buying even if we get hosed on price?.

Interesting question on the 9/10ths unless that's something to do with the Federal Taxes?
How were CA blackouts and greed connected? My understanding was the demand had exceeded supply since regulations were so difficult no one built power plants in CA and the out of state supply is limited and regulated as well. If the utility can't serve more demand since they can't build profitable supply sources due to the laws then you have just imposed a regulatory cap on the supply and so something has give.


I think Enron was responsible for some of California's brownouts/blackouts. I'll try and find some links later. Maybe someone else can chime in.

As for the 9/10 ths pricing, that is simply marketing. When gas stations are competing based on a few cents difference in price, $2.94 9/10 looks better than $2.95.

Edit: added link

California Energy Crisis

The EIA has their discussion of summer gasoline prices up in This week in petroleum. (Link works now, but will probably be to a different TWIP later.) According to it:

Over the past five years (2005-2009), regular gasoline retail prices have increased an average of 18 cents per gallon from April to May, when most of the country is required to switch to summer gasoline. However, in EIA’s May Short-Term Energy Outlook the increase is expected to be just 7 cents per gallon this year. Projected refinery utilization rates that are below seasonal norms, high inventories, and relatively low year-over-year consumption growth are key factors that are expected to keep this summer’s 3-2-1 crack spreads below the previous 5-year average.

Clearly it also makes a difference where oil prices are headed. If oil prices are down (because of financial problems and increased recession), gasoline prices will not be as high. The many people laid off from work will still have trouble buying gasoline, as will some of those with furlough days.

For those who were thinking "peak oil = high gasoline prices", this is sort of disturbing--somehow, there is a belief that oil prices have to be higher, if less oil is being pulled out of the ground. But if the problem is, "high oil prices -> recession", and "high oil prices-> loan defaults -> cutbacks in credit availability", then we get the situation we have today, with oil prices that are high compared to say, 10 years ago, but not high compared to what we had a couple of years ago. It doesn't take very high oil/ gasoline prices to have a recessionary impact on the economy, and we seem to already hitting those prices with oil only in the $75 -$85 range. It is not clear that gasoline prices will ever be $10 or $20 gallon (or even $5 gallon).

Clearly it also makes a difference where oil prices are headed.

That is actually a point I stressed in the most updated version of this:

So far in 2010, gasoline inventories have been at very healthy levels. While some inventory draw down can be expected during the transition to summer gasoline, it is a pretty safe bet that the current high level of gasoline stocks will prevent a rapid escalation of prices this spring. I would expect no more than a mild price increase between now and summer, and at the current inventory levels it would not be surprising to see prices start to decline from present levels. However if oil prices escalate, that could trump high gasoline inventory levels. This is why gasoline is presently about $1 more than it was last year at this time; oil prices were $30-$40 lower than they are now. But that's a topic for a future essay.

I watch two things relative to gasoline price predictions: Oil prices and gasoline inventories. They can tell you where gasoline prices are headed in many cases.