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I only see a 1MPG increase from midgrade to premium, however the car seemingly has a bit more power especially at low RPM with premium. I prefer how it feels with premium so I'm willing to pay a bit extra even if the cost per mile doesn't work in my favor. Here the average is 20 cents more for midgrade, 60 cents for premium.

My car will never get 87 octane again while I own it, low RPM(below boost) seems to have less power and if pulling out into traffic seems much more dangerous. At minimum I would run midgrade.

The DIC(drivers information center) is optimistic, so if your seeing 38mpg, that's probably only 34-35mpg pump calculated.

 

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Filled up after my drive in. 36.6 mpg calculated for my first full tank. DIC showed 37.4. Not as much of a difference as I thought.

It will very from tank to tank how far off it reads, Mines usually between 1-4MPG high(around 8% optimistic on average). Example, my last 5 fillups pump calculated vs DIC:

Pump 37.36MPG DIC 40.8MPG = 3.4MPG high
Pump 38.98MPG DIC 42.5MPG = 3.5MPG high
Pump 41.22MPG DIC 44.7MPG = 3.5MPG high
Pump 41.37MPG DIC 43.3MPG = 1.9MPG high (85% highway)
Pump 36.97MPG DIC 40.4MPG = 3.4MPG high

I recommend setting up a fuelly.com account to keep track of your milage.

Very interested in what difference I see from the premium gas.

Make sure to run premium a few tanks before you make any judgements, seems the car needs some time adjust fully to the higher octane.

 

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Thanks. I did setup the fuelly account--apparently I did not set up my signature correctly to show it on my posts.

I beleive there is a thread that goes over this in better detail, but here is a quick run through that might help getting your signature setup.

Goto your fuelly page, the one that has all your fill up logs listed. Just below the logs in the lower left, is a link that says "badges". Click on that link. Once on the Badges page copy the text in the first box.

Return to Cruzetalk, Click on settings in the upper right, Then in the left column select edit signature. Paste the text you copied from Fuelly into the box then click the save signature button.

 

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We almost have to factor in the fuel rate of flow at the pump when calculating our mpg.

I experienced this on my last two fill ups. First one was much lower pump calculated than I thought it should be, the DIC was also reading 6mpg high. Next fill up though pump calculated seemed much higher than I thought it should be & the DIC was only 2mpg high.

Seems that first fill up the pump did not kick off at the same point as my normal station & I got extra fuel.