Where I live gas is at $2.50 at 10% Ethanol at 87 octane and $2.90 for non-ethanol at 89 octane per gallon. I understand that smaller engines do not do well on ethanol. Is the loss of efficiency from containing ethanol worth the saving? Is the cost difference between the 2 either way worth the mileage and maintenance? Does smaller the engine equal less efficiency on ethanol?

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    What are you considering a "small engine"? To me a small engine is used on a lawnmower. Commented Sep 13, 2018 at 21:00
  • Where do you live? As a resident of the UK I'm quite horrified! Our pumps provide 95 octane as standard and between 97 and 99 octane performance fuel. Race fuel (available on the pump at a small number of filling stations, usually near race tracks) is 102 octane. Most European vehicles specify a minimum of 91 octane fuel. Commented Sep 18, 2018 at 8:26
  • The octane number in US is (MON+RON)/2 and the octane number in EU is RON. RON is higher than MON (but not by a fixed constant offset), and therefore, you can't directly compare the octane numbers in US and EU. Usually the difference is perhaps ~5-~6 units.
    – juhist
    Commented Nov 2, 2019 at 8:40
  • Where in the US do you have the option on ethanol free gasoline ( except av-gas) ?When I could still get ethanol free gasoline in some locations , I generally got 10% lower mileage with the ethanol-gas instead of the theoretical 3 % loss. Commented Nov 2, 2019 at 17:43

2 Answers 2


small engines don't need the extra octane and the extra octane won't produce any higher efficiency. However, non-ethanol gas doesn't attract water link ethanol gas, so it doesn't corrode the inside of the carb. That's why it's preferred. But you still need to add fuel stabilizer if you're going to keep it in the tank more than a month.


Consider this:

What costs $2.50 per gallon of E10 costs $2.50 / 0.9 = $2.7778 per gallon of actual gasoline in it.

So, if you're wondering if the smaller energy content of ethanol is an issue, don't! Even if the ethanol had actually no energy content in it, the E10 would be cheaper per unit energy.

In reality, the energy content of ethanol is perhaps around ~70% of that of gasoline. So you're paying $2.50 / (0.9 + 0.1*0.7) = $2.5773 per gallon of gasoline equivalent.

The slightly increased octane number won't even in a perfect engine that is capable of using the extra octane create much difference. Certainly it won't make the $2.90 gasoline better than the $2.50 E10 by more than the cost difference.

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