I have been lurking for several months. I agree to the consenus that ethanol kills MPG. My normal drive is 15 miles of 50-60 and then 45 of Interstate drive each day. I saw this AM how much cold air (30) will kill MPG, it made almost 5 MPG difference over the same stretch of road from this AM to the PM this afternoon.

My question is, if you had no choice but to get Less than 10% Ethanol at a station, would it be benefical to get a higher Octane to offset the Ethanol? We are going to Disney this weekend and all the Hess Stations on property all have the stickers "Less than 10% ethanol". But sometimes, their Md Grade is cheaper than Regular off property.

Maybe some day soon, I will understand enough to get a scan gauge and know how to use it

 

Short Answer - NO

Octane is widely misinterpreted as a measure of energy in the fuel. In reality it is a measure of resistance to preignition so a high octane fuel does not ignite as readily as a low octane fuel and often actually has less energy per gallon. Why is high octane fuel considered "high performance"? Because it lets an engine operate with higher compression and more ignition advance, which increases power but will result in higher gas consumption.

Pure ethanol supposedly has an equivalent octane rating of over 120.

 

Pure ethanol ( most people can't get it ) has an octane "rating" of 115 actually. E85 has an octane of 105.

Octane is actually used to BOOST octane. Only 5.75% is required to replace MTBE in any area that uses oxygenates.

Thus, the lowest octane will normally will have the least ethanol.
In the Mid-west, mid-grade has the most ( a full 10% ) and out here, and 87 and 89 octane are the same price.

 

KenG's description is accurate. A fuel's anti-knock characteristics are measured and assigned a number that represents how those characteristics compare to a mixture of iso-octane and heptane. Fuel rated at 87 has a resistance to preignition similar to a mixture of 87% iso-octane and 13% heptane. This is a simplification and there's more to it than that, as there are two generally accepted measures of anti-knock which return different results depending on the test. But this is a good layman's way of understanding the idea.

100% ethanol is more knock-resistant than 100% iso-octane, which is why E85 has a rating higher than 100.

For fuel economy, note that that fuels with high octane ratings contain LESS ENERGY than lower rated fuels. Engines burning high octane fuel compensate for this lack of energy by burning more fuel. For example, when the computer in your flex fuel vehicle reads that you're running E85 it just pumps more fuel in to make up for it. Hence the consistently lower mileage when running E85.

Specifically talking about E85: It has about 1/3 less energy than gasoline and at least 1/4 less than E10. That's why you must burn 25%-35% more to drive the same distance. And that's why E85 must cost at least 25% less than gas to break even on it, notwithstanding the feel-good factor of running it. Speaking strictly economically, gas at $2.80 per gallon requires E85 at less than $2.10 per gallon to make sense in a flex fuel setup.

It's actually pretty easy to decide what to run. In Brazil where everything is E85 and they make ethanol from sugar cane, drivers all know how to tell what to buy. When they get to a station, if E85 is 2/3 the price of gas or lower, they buy E85. As soon as it goes higher than that, they know the mileage drop will offset any savings and they switch to gas for the better fuel mileage. It's remarkable how they have adapted.

RE: The original question: When it comes to commercially available motor fuel, energy content drops as octane rating rises. The higher the rating, the "weaker" the fuel. Choosing a higher octane fuel like E85 or premium gas at the pump will almost almost always decrease, not increase, fuel mileage.

YMMV, but that's how it works in a fixed setting.