E85 fuel—a mix of 85 percent ethanol and 15 percent gasoline—rapidly fell out of favor. But 98 percent of US gas stations offer E10, a blend of 10 percent ethanol and 90 percent gasoline, according to the Environmental Protection Agency. At this concentration, the ethanol oxygenates the fuel and increases its octane rating; it also stretches the country's supply of gasoline by diluting it.

As the name suggests, E15 is a mix of 15 percent ethanol and 85 percent gasoline. It's much less common in the US due to the EPA's restriction on summer sales (because of the greater evaporation during hot weather), but in 2019, former President Donald Trump approved its use year-round. In 2021, the US Court of Appeals for the DC Circuit ruled that the EPA had exceeded its authority in doing so. //

It's also bad news for anyone concerned about climate change. Although biofuel blends were supposed to save us, growing energy-intensive corn to dilute gasoline is probably worse than just burning the gas itself, as Ars' Tim de Chant covered in February. Last week, the Intergovernmental Panel on Climate Change published its latest report with the finding that we have to massively cut carbon emissions within the next three years if we want to have a chance at limiting warming to even just 2°C globally.