For multiple reasons, the eco-friendliness of FFVs is questionable. Cost savings are dubious, too. And as it turns out, it's not necessarily consumer demand or sustainability concerns that are driving the proliferation of FFVs.
Let's start with the basics. There's no change in vehicle performance when you use ethanol. You won't notice a difference in acceleration or horsepower.
The key differences are energy density and CO2 (carbon dioxide) emissions. Lower energy density just means that there's less energy in a gallon of ethanol than there is in regular gasoline, so you have to burn more to travel the same distance that you would with gas.
This, in turn, means you have to burn more fuel to get moving, and your vehicle actually releases more CO2 than it does with gas. However, because that CO2 was derived from a plant source instead of petroleum, you're (in theory) simply releasing it back into the atmosphere, where it would have gone anyway as the plant naturally decayed. Still, the eco-friendliness of FFVs is, at best, unclear.
Fuel efficiency is another thing altogether. As with all things related to mileage, there's a lot of variability here, but mileage per tank drops between 15 percent and 30 percent when you switch to ethanol. And because ethanol fuels are generally about the same price as gas, you might actually spend significantly more money -- perhaps hundreds of dollars per year -- to get around on just ethanol.
Considering these facts, you may wonder why anyone would build or buy FFVs. In the United States, federal Corporate Average Fuel Economy (CAFÉ) standards dictate that auto makers must offer vehicles with a minimum gas mileage of 27.5 mpg (22.2 for light trucks); otherwise, they face stiff fines.
There's a loophole, though: If those companies build vehicles that burn ethanol, they won't be fined nearly as much. Basically, it's a government strategy to encourage the use of more ethanol, which effectively subsidizes American crop farmers and could potentially reduce American dependence on foreign oil.
As for the flex engines themselves, well, they're a mature technology that requires zero skills or adaptation on the part of consumers. All you need to do is find one of thousands of gas stations that offer E85 fuel, and your flex engine will take you as far as you want to go -- just not quite as far as it would if you filled up with gas.