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.

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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.

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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.

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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.

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Winter blend is coming. Prepare yourselves.