Accidentally pumped some mid-grade instead of premium

[snowtown_71]

I just bought a 04' G35 the other day and went to fill up my tank today. I accidentally put about 3 or 4 gallons of mid-grade unleaded before I realized it wasn't premium (this gas station didn't have premium on the right of the pump like the most gas stations) and put another 4-5 gallons of premium in the tank. I didn't want to top it off because I wasn't sure if it was going to help. Felt like a total dumba** and was wondering if it will cause any damage or do anything to the engine. From what I can tell and was researching, the anti-knock system will help prevent the loud knocks and my fuel economy will be less. But should I worry? I was thinking about siphoning most of the gas out and then refilling it with the premium. Is that a knee-jerk overreaction? Trusting the forum to help me out and to not lay into my stupidity. Thank you

[United EC]

Dont stress too much. Your anti-knock sensors should take care of your regular gas matter. Gas in todays world is so packed with ethanol it barely makes a difference. Having premium fuel will make you have better fuel milage and better power. I would just make sure I burn through that tank and make sure I run premium all the time. Hope that helps!

[Kendahl]

Does your owner's manual specify premium only or does it merely recommend premium? If it just recommends premium, you could run 87 indefinitely and never put in premium. Telcoman did this with his G35 and had no problems. Even if the manual does insist on premium, it also says that you can use part of a tank of lower octane fuel if that's all you can find. Top off with premium as soon as you can and drive gently until that tank is used up.

[joe603]

Like the guys posted, the ECU will adjust according to the Octane content in the fuel. If you have less, the engine will have less output (HP).

[jmann]

If your premium is 93 octane, you can safely mix 10 gallons of 89 octane with it and still have 91 in the tank. All midgrade is just a blend of regular and premium gas.

[Kendahl]

If your premium is 93 octane, you can safely mix 10 gallons of 89 octane with it and still have 91 in the tank. All midgrade is just a blend of regular and premium gas.

Mixing different octane levels to get an average is correct. However, not all midgrade is a mixture of regular and premium. Where I live, midgrade (89 octane) is regular (87 octane) with 10% ethanol. A few months ago, "regular" became 85 octane with 10% ethanol. At most stations, even premium now contains 10% ethanol.

[larsG35]

At the basic principle of it, you could consider pure ethanol to be a fuel with ~100 octane rating (AKI), but lower energy density than the typical constituents of "gasoline". It is a pet-peeve of mine that it is becoming common practice to dilute any grade of gasoline with up to 10% Ethanol (the highest legal amount) because you will get lower MPGs with it. Ethanol is just a lot cheaper to add than the aromatic molecules that typical boost the AKI.

So here is my public service announcement to the NICO community. If you have a choice between 10% Ethanol added and 0% Ethanol added, make sure the 10% fuel is at least 4 cents/dollar cheaper. (0% is $1.00 = 10% is 96 cents; 0% is $3.00 = 10% is $2.88) Otherwise you are actually saving money by getting the more expensive 0% ethanol added because it has higher energy content.

-and E85 needs to be ~26 cents/dollar cheaper to be worth it... if any of you drive trucks I guess.

[joe603]

Where I live, if you want gas without ethanol, it's 50-60 cents more per gallon.

Here's a cool website for it: http://pure-gas.org/index.jsp?stateprov=GA

[jmann]

If your premium is 93 octane, you can safely mix 10 gallons of 89 octane with it and still have 91 in the tank. All midgrade is just a blend of regular and premium gas.

Mixing different octane levels to get an average is correct. However, not all midgrade is a mixture of regular and premium. Where I live, midgrade (89 octane) is regular (87 octane) with 10% ethanol. A few months ago, "regular" became 85 octane with 10% ethanol. At most stations, even premium now contains 10% ethanol.

I left out mentioning the ethanol factor because it does complicate matters, most gas around here seems to have 10% ethanol these days. Still, being 10% across all tiers means the regular and premium gas base without the ethanol uses some aromatic additives to make the AKI index higher in premium, although not necessarily 93 octane. 93 octane with 10% ethanol is probably just 91 octane before the alcohol is added. Either way the blend is the same. regular is mixed with premium to achieve midgrade. Likewise, 10 gallons of 89 octane mixed with 10 gallons of 93 octane has a net AKI of 91.

[audtatious]

I run midgrade most of the time in the '07 sedan. No issues.