Can I mix 93 and 89 octane?

My 2009 Honda Civic Si says that a minimum of 91 octane fuel is required. However, the gas stations I have visited do not sell 91 octane. Can I mix the 93 and 89 octane fuels to get an average of 91 octane? (I realise I could just fill with 93, but if I want to save a little money by filling half with mid-grade instead of premium?)

Yes, feel free to mix octanes, as well as brands of gasoline.

Yes, it’s similar to your wife mixing 3% milk and 1% milk in equal parts to get 2%.

Years ago,Sunoco sold at least 5 different octane levels. However, there were only two tanks. The lowest octane came from one tank and the highest octane from the other tank. The grades in the middle were a mixture that was done by the pump. You are duplicating the same process.

It might be cheaper to mix 87 and 93. Crunch the #'s. I once had an Accord with a highly modified engine, and that’s how I filled it with ~90 octane to stop pinging.

You’re exactly right. Around here, they charge $0.20/gallon more for 89 than 87, and $0.30 more for 91 than 87. Just based on the price of 87 and 91 it should only be $0.10 more for 89. So OP would be better off just mixing 87 and 93 50/50 (90 is just about no different than 91), and they’ll save money. My brother works in refining, and he comments on how folks overpay for ‘mid grade’.

I don’t see why you’d want to have that much of a hassle. You bought a performance vehicle, why skimp on the fuel to feed it?

What about a simple fuel additive? Wouldn’t that be more cost effective?

You could, but the savings would be…slight. The windfall $1-$2 savings would not be worth it to me. Just use 93 octane and forget about it.

Octane rating is actually a measure of how explosive the fuel is. A less explosive fuel can be compressed more without exploding prematurely. Premature explosion (knocking) is bad.

Higher compression (and therefore more efficient) engines require a less explosive – higher octane – fuel. Mixing two different gasolines may or may not end you up with a tank that is half way between the octane ratings of the two.

Your engine should run more or less OK on 89 octane because the computer should retard the timing a bit to keep it from knocking. But that costs you power. That’s the theory anyway.

My guess is that the small mileage improvement from using 93 octane will cover the small difference in price between 93 and 89 octane. So why not just use 93 octane?