I have a 2012 F-250 Ford built to use flex fuel. What is the best way to maximize miles per gallon? Is there any advantage to running 1/2 tank of 93 premium and half flex fuel? Should I run strictly flex fuel and an octane booster? Power is not an issue. I drive a lot of highway miles daily. Thanks for your time, I enjoy your postings. — C.S.

Jeff Smith: This is a great question. When you say flex fuel, for the sake of this discussion we will assume that you mean E-85 which is 85 percent ethanol and 15 percent gasoline.

As most of our readers are probably aware, ethanol produces only about 75 percent of the heat as the same amount of gasoline. Therefore, to make the same power (or to drive the same distance) the engine must burn roughly 25 percent more fuel. This has been shown to be the case in several tests over the years.

A flex fuel engine must be designed to operate on both gasoline and ethanol under a wide range of mixtures. The big issue really comes down to octane rating. We need to start with a few definitions so that everybody is on the same page. The American Society for Testing and Materials (ASTM) recently set a standard — D5798 for the sticker E-85 on the pump that allows this fuel to contain anywhere between 51 percent to 83 percent ethanol. That’s right, E-85 only has to be as little as 51 percent — depending upon the geographical location and season. This is because during winter E85 needs a higher percentage of gasoline in order to light better in cold weather.

Your flex fuel truck uses a sensor that measures the percentage of ethanol in the fuel and changes the amount of fuel injected into the engine to compensate for the percentage of ethanol in the fuel. The lower percentage of ethanol — let’s say 51 percent — may at first sound like a bad deal. But in reality it will probably improve the fuel mileage for a couple of reasons. Straight ethanol (often referred to as E98 because a pure ethanol is difficult to make) has a very high octane rating — 105 is a common reference number.

As ethanol is mixed with gasoline, it raises the base gasoline’s octane rating. It turns out that at first, a small amount of ethanol mixed with gasoline creates a big increase in octane but as higher percentages of ethanol are included (above 50 percent), the benefit decreases. This just follows the law of diminishing returns.

So with E30 — which is 30 percent ethanol, the octane rating is around 95 octane which is better than any “straight” E10 premium.

Keep in mind that more than 90 percent of all pump gasoline sold in this country is already mixed with 10 percent ethanol. There is great confusion over the move to E15, most of which is fronted and pushed by the American Petroleum Industry in the form of scare tactics that claim that most cars cannot run on that additional 5 percent of ethanol. Increasing the ethanol content to 15 percent may create minor issues for older, carbureted engines but a 5 percent increase of ethanol that already has 75 percent of the energy value of gasoline, isn’t an issue with most cars.

The great benefit is the octane increase when the ethanol percentage goes up.

With E50, for example, this is roughly equivalent to a 98 octane fuel.

So your question of whether to use an octane booster is of no concern. With E50 you already have far more octane than your engine needs, but this is not a problem.

Because E85 has such an excellent mixing percentage I would suggest changing your idea of running a half tank of 93 octane with a half tank of E85.

Instead, I think you could save the cost of the high octane fuel. I’ve included a mixing ratio chart that was calculated using Wallace Racing’s online calculator. This assumes mixing E85 with 89 octane gasoline that is already an E10 fuel. In the last example, mixing 5 gallons of E85 with 10 gallons of normal 89 octane gasoline produces the equivalent of E35 that has an octane rating of 93. The best part is that since E85 generally costs much less than gasoline — you have a fuel with a higher octane than premium gasoline but at a lower cost!

Gasoline 89 Octane
E10 (Gallons)

While E85’s detractors will point to reduced fuel mileage with E85, some recent studies produced by the ethanol council has found that increasing the ethanol percentage from E10 to as much as E30 can actually improve the fuel mileage on some vehicles. This will sound counterintuitive but there is a solid reason for this. Generally, this improvement is found in small displacement, turbocharged vehicles where the engine works harder. With a higher octane fuel like E30, the engine does not experience detonation so the higher octane allows the engine to run with more ignition timing, which improves efficiency and the car experiences better fuel mileage.

There’s also a different and perhaps better way to look at fuel mileage. As you can see in the accompanying photo, as the blender pump adds ethanol in different percentages, the octane increases while the price per gallon decreases.

(Image/Jeff Smith)

So if we calculate a cost per mile — which is really what we should be concerned with — then fuel mileage can decrease but it’s still less expensive to drive in a cost-per-mile basis. Let’s do a calculation and see how this works.

Let’s use 20 miles per gallon as our gasoline standard and say we pay $3 per gallon for the 87 octane E10 fuel.

Dividing 300 cents by 20 mpg equals 15 cents per mile.

Now let’s project we lose 1 mpg by using E30 gasoline so now we’re getting only 19 mpg. But that higher octane E30 blend only costs $2.75 per gallon. Do the math again and we find that our cost per mile is now 14.5 cents per mile. While the fuel mileage decreased — the cost per mile was reduced and that’s all that should matter.

This may not happen with your 2012 F-250 with a 6.2L flex fuel engine but it is possible that the added octane from an E30 or E35 mixture could produce potentially better mileage by allowing the engine to run with its optimized ignition timing. Based on this data, it seems that it would be best to test an E30 to E35 percentage and see if this at least maintains the same fuel mileage or perhaps only loses a small percentage. As we showed in the above calculation of cost-per-mile — it’s possible to lose mileage but reduce your cost per mile. That’s what it’s all about.

Keep in mind that because the standard for E85 is now a broad range of ethanol from 51 to 83 percent, the ratios listed below in the chart would change if the pump E85 was really only 70 percent.

These percentages will likely change with the seasons — with summer time where the ethanol percentages will be highest. This might sound a bit complex, but it’s possible that you can have better power, higher octane, a cleaner fuel, and still maintain a reasonable cost-per-mile. That would make it a winning combination.

Share this Article
Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.