[chevrasaki]

My personal recommendation would be to fuel the car with 91+ octane.

As you've noticed the Cruze computers are sensitive to knock and will retard engine timing to protect itself if it senses pre-detonation. Knock is caused by the fuel exploding too early in the cylinder due to high temperatures, and high pressures. Since the Cruze is turbo charged, it's a check mark for high pressure. The intake temperatures vary wildly depending on conditions. But basically you're going to reach high temps at least a few times each tank of fuel. The exception is in winter when the temps are always cold.

I've toyed around with various grades of fuel and have settled on 93. I get better fuel economy with premium, which is almost enough to completely offset the price increase. Also, when I run 87 my car is very quick to pull timing and as a result it feels very under powered. Just feed it premium. It's not like it chugs gas. A car is generally your second biggest purchase, no sense in fooling around with mixing fuels it's not recommended to run just to save a few dollars. If you really want to save money on fuel, buy a 50cc scooter on craigslist, they get 110+ mpg and I wouldn't feel the least bit bad for filling it with 87 octane. Or bicycles I hear are an option for some people. Much cheaper.

 

[chevrasaki]

I've noted that many premium blends, Shell, BP, use nitrogen based boosters, causing more violent combustion, and the engine to vibrate more, while E85 uses a much more slow burning, better suited for low revving cars like the Cruze.

I've always felt like my engine would wear quicker when I use hi octane fuels from BP and Shell, for this purpose.

Well I can't help how you "feel" about it, but the owners manual recommends using Top Tier fuel for engine longevity. By ignoring the engineers that designed the vehicle, and deciding to make your own uninformed decisions might end up shortening the life of the vehicle. The use of E85 will void your warranty because it is listed under the "Prohibited fuel" section of the manual.

My final thoughts: Do not run E85 in the Cruze. Do not fuel with cheap off-brand fuel from low volume gas stations. 87 octane is OK to use but will feel under powered due to decrease in timing. 89+ is preferred. And I still personally recommend 91+ for the best fuel economy and greatest knock resistance. You are your own person capable of making decisions so good luck with your choice, whatever it is but I have a feeling you've already made up your mind.

 

[chevrasaki]

I decided to fill up with 93 Shell V Power to see what the difference is, and since I had 65 cents off with Fuel Rewards Network figured I'll try it. Only about 30 miles in and maybe just in my head, but it seems like accelerating is a little faster/smoother. Prem is around ~80 cents more compared to 87. I have only filled up 3 times with 87 and my average is ~34MPG when calc fuel vs miles. The car reading is a little off since it told me 37.1. Be interesting to see if I see any improvements with 93 on MPG, I might switch back to 87 since I m thinking I will not get more milage to make up the difference in cost and will just see a boost in my head. Would just filling up 1 time with 93 would I see MPG improvements or would I need to use 93 for a few tanks to really see something?

Brace yourself, this could get boring.

It may take a few tanks to get the full benefits of a highly knock resistant fuel like 93. The car has knock sensors and will "pull" or retard the timing on the engine if it senses knock (aka pinging or predetonation). Knock happens when you put the air/fuel mix in a condition where it spontaneously combusts before the spark plug gets a chance to ignite it at the optimal time. When the air/fuel detonates too early, it can harm the engine. The cruze has timing tables for fuel as low as 87 octane so you can safely run cheap gas but there are side effects. The 1.4L engine in the Cruze is turbo charged so the air pressure going into the cylinder is higher than naturally aspirated cars. When you increase pressure of a gas, the temperature increases. Some of that extra heat is removed by the intercooler. But since the intercooler is sandwiched in between the coolant radiator and the A/C radiator, it will often heat soak in the summer, in traffic, or with A/C use. That's when the air intake temps skyrocket and cause knock, so the computer retards timing to protect itself which results in decreased power. Not a huge deal, but since the Cruze is heavy, and doesn't have a lot of power to begin with, now it has to use more fuel hence the decreased economy. Depending on how you drive, you may not notice it. But if you have a lead foot, you'll lose a pretty significant amount of MPGs running 87.

You can kind of get away with running 89 or 87 in the winter because the intake temps stay low. When air is cold, it's dense and more potent. Usually that means you can get more power, but the Cruze is torque limited so it uses the boost levels to control how much torque it makes. Since it can easily achieve the 148 ft-lbs in the winter with that delicious dense cold air, it doesn't need as much boost so the pressure is lower, which means the temps are lower, and since the winter air is already a lot colder, the air fuel/mix is a lot less likely to pre-detonate so the computer doesn't need to retard the timing. You'll still get less fuel economy in the winter no matter which grade you run because they have to blend in extra chemicals to keep the fuel from freezing, and it also takes the car a lot longer to reach operating temperature so it runs rich more often in winter.

Hope this helps.