What Is Flex Fuel?

Flex fuel refers to a gasoline-based fuel that does not solely consist of gasoline, but instead a mixture of gasoline and another fuel — most commonly ethanol. Cars, trucks and SUVs with engines that accept blended fuels are commonly referred to as flex fuel vehicles. In this article we'll break down what flex fuel is, what it takes for your car to use it, and tell you where you can get flex fuel.

So what is flex fuel?

Flex fuel is made up of a blend of gasoline and another fuel (typically ethanol but vehicles that run on a gasoline/methanol combination have also been tested).

Ethanol is an alcohol-based biofuel typically derived from renewable sources such as corn, sugarcane or switchgrass, but corn kernel starch is the most common source of ethanol in the United States. Ethanol is typically seen as a more environmentally friendly fuel because it cuts down on greenhouse gas emissions and helps cut dependence on fossil fuels.

The gas/ethanol mixture is known as E85, and according to the U.S. Department of Energy, there are more than 4,200 public E85 stations in 44 states across the country. For E85 fuel, ethanol amounts to 51% to 83% of the blend, depending on the time of year and geography. In this context, you can think of pure gasoline as E0. Flex fuel vehicles can run any mix of these blends up to E85. It also means you have to think less about what you put in your tank. Why? As we'll see shortly, flex fuel vehicles can handle any gas-ethanol blend (up to 83% ethanol) entirely on their own.

What makes a flex fuel car?

Cars with engines that can use different ethanol-gasoline blends are flex fuel cars. The engines themselves aren't very different from any other car's internal combustion engine, with the major changes coming down to modifications to the car's fueling system.

The oxygen sensor is one of the keys to making the E85 fuel work seamlessly. All cars have oxygen sensors, and in a flex fuel car its job is to monitor oxygen content of the gases flowing through a car's exhaust system. Depending on what it reads, it will either enrich (add more fuel into the cylinder before combustion) or lean out (add less fuel into the cylinder) the air-to-fuel ratio inside the combustion chamber. It will also change other parameters like the ignition timing inside the cylinder to maximize both power and efficiency made by whatever fuel mixture is in the car.

One of the nice things about flex fuel cars is that they handle the mixing of fuels seamlessly in the background while you drive along. There's nothing you as the driver have to do in order to make the various blends of fuel work inside your engine. So whether you put gasoline, E10, E85, or any mixture of gasoline and ethanol in the tank, the car itself will work out the best mixture of fuels for its performance. Service intervals for fuel flex engines are largely the same, and while making sure the fuel system is working optimally is important, your local dealer should be equipped to handle its maintenance and care.

Where can you get flex fuel?

Flex fuel is available at more than 4,200 gas stations in the country, and stations that sell ethanol blends will typically have a yellow sign out front that signals which ethanol blend they sell and its price per gallon. If you want to specifically fill up with flex fuel, the U.S. Department of Energy has an interactive map that allows you to filter fueling stations that sell flex fuel via ZIP code here. It's worth noting that running your car on an ethanol blend may result in worse fuel economy, but E85 is typically much less expensive than gasoline. As a result, this may be a trade-off worth making, but ultimately it comes down to personal preference and the availability of ethanol in your area.