All cars made since 1981 have been able to use 10% ethanol without damage.
10% ethanol has three main benefits: it burns more cleanly in the engine, it produces less exhaust pollution, and it boosts the octane rating by two points.
But it has one disadvantage. The energy per litre is a little lower. Using E10 fuel will reduce your range per litre by about 3%. So the price needs to be more than 3% cheaper for it to provide a cost benefit. A reduction of 12 cents per litre therefore provides a good cost saving.
Do NOT use a higher percentage of Ethanol than what's available at the pump for your vehicle.
The reason higher Ethanol mixes (e.g., E85) are used by higher performance vehicles that have been tuned to use such a thing, is that Ethanol has a lower calorific value - meaning it's much more stable, temp-wise, given more pressure and heat. That's why it's highly preferable to pump gas when tuning a vehicle and adding more boost or spray. The more stable the fuel, the less chance of uncontrolled detonation.
The problem with using a fuel that's not rated properly for your vehicle (unmodified) is that the latter was designed to burn fuel given certain conditions. (compression, air-fuel ratio, combustion timing, etc.) All else remaining equal, you introduce a fuel that won't burn as easily and you might as well put watered down pump gas in it: same effect. Or, who knows. Improper detonation, unstable idle, engine gunk, really bad emissions, and a host of other things - not the least being a check engine light goes off instantly and you void your warranty.
Last two bits of info:
1. Gas companies or distributors usually mix in their own brand of fuel "detergents" that increase the stability of fuel. These chemicals are, ultimately, what give your car the higher RON rating at the pump. (87/89, 91, 93.) The higher the rating the more stable, but it doesn't mean that your car needs it. As you might guess, higher RON fuel are what higher-performance engines like that of the Evo or V6 Eclipse require from the factory, as these are tuned far more for performance output per volume displacement than, say, the Eclipse Cross.
2. As ribuck mentioned, petrol with a mix of ethanol usually produces a lower amount of heat, and a cleaner burn than the latter. But, it produces less power. I remember, when gas companies started rolling these out en masse in the US in 2007, I took a cross-country drive in my 3.8L Eclipse where I started out with E-10. Got about 380mi a full tank on cruise control. I gassed up somewhere in TN where, at the time there were still pumps with 100% gasoline. Next fill-up was at around 410-420mi. It was significant.
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