What You Should Know About The Fuel You'll Be Putting In Your Car Soon

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You know how for some of the liquids you pour in your body there are seasonal variations? Lots of coffee chains are as we speak pumpkin-infusing everything they can get their hands on, for example. Liquids for your car are the same way — we're entering the time of Winter Blend gasoline now. Here's how it's going to impact you and why this is happening.

Unlike other seasonal fluid variants, like McDonald's Shamrock Shakes, the changes to winter blend gasoline aren't just green food coloring and the slightest touch of LSD. There's very real differences between the gasoline you've been buying all summer and the petrochemical you've been getting (in most places) for about a month now.

There's a number of differences in winter blend gasoline, but there's really two major results of these changes that you're likely to note, and they're both caused by the same thing. The first thing most folks realize is that gasoline tends to be cheaper in the fall and winter months. Some of this is due to a drop in demand, but the truth is that the gasoline blend used in those months is actually cheaper to produce.


That same cheaper blend also has one other major characteristic: it will boil at a lower temperature, or to be more technical, it has a higher Reid vapor pressure (RVP). RVP is the vapor pressure of a substance at 100°F. Air has a vapor pressure of about 14.7 psi. If a liquid has a RVP higher than the surrounding atmosphere, it boils. Think of it as that pressure overcoming the weight of the air pressing down on it and bursting through, into a gas.

Both of these factors are due to one thing: butane. That's right, the bastard gas.

Butane, like a true bastard, is cheap. While butane can be used as an internal combustion engine fuel, its RVP is a whopping 52 psi, which means it boils like crazy in our normal atmosphere. Which isn't ideal for easy fuel storage.

This also means that even if you add only 10% of burnable, cheap butane to a gasoline mixture, you increase the fuel's RVP by 5.2 psi. And, when you're trying to keep the overall RVP down below that of air, that's just too much, especially in the summer, when temperatures inside the gas tank can meet or exceed 100°F.

Normal summer gasoline can only have an RVP of between 7.8 and 9 psi, depending on location. That means only 2% cheap, flammable butane at best. Having your fuel turn boil off into a gas is bad for a number of reasons, emissions controls being one of the biggest, since you probably want to minimize the amount of gaseous fuel you're breathing in when you're wandering around a parking lot.


In the winter, though, things change. Temperatures are lower, increased bulky sweater-wearing helps keep the atmosphere from becoming too sexually charged, and gasoline can up the butane content from around 2% in the summer months to about 10%. As a result, gasoline becomes cheaper to produce, and the very altruistic oil companies pass the savings on to you.

That's the good news. The bad news is that butane, as a cheap, bastard gas, doesn't have that many carbon molecules in it: four. Gasoline mixtures typically have carbon chains between 6-10 carbon atoms long. More butane in this mix will push the carbon count to the lower end of that spectrum, and (I'm oversimplifying here, but still) more carbons is more energy.


Sure, it's a bit more volatile as well, which could help with winter starting if you drive something like a 1907 Franklin. For most cars, starting won't be affected at all, but you'll likely notice a 2-8% drop in fuel economy. So, even though the gas costs a bit less, it's really pretty close to a wash (or worse) because you'll be using more of it.


Winter blend is coming. Prepare yourselves.