[Ma v e n]

You have a brand new Cruze with wide heavy wheels and tires, and a subwoofer, that you drive on short trips exceeding the speed limit....That's what's causing it. In addition to the issue of weather or not your DIC was ever reset.

 

[Ma v e n]

Sorry, I forgot that some of the RS cars had small wheels, my Redline has 18s.

The DISC is driver information center. If it hasn't been reset from before you started driving it could be displaying lower than actual MPG. It's very common for cars to come off the dealer lot with single digit lifetime fuel economy.

The big culprit is the short trips with multiple stops.(the higher speeds don't help either)

My hatchback has the bigger spoiler, the bigger wider wheels and tires, and on a commute of 21 miles, I have a section that's 14 miles long uninterrupted and I run at 50mph. Car LOVES that, I routinely see 50mpg+ averages on that road. Add back in the mpg from my slower neighborhood area with 5 stops signs(about a mile of 30mph) and a few miles at 65mph and another stop sign or three and my mileage for any given day drips down to 38-40mpg.

Lifetime average on my car at 15000 miles is I believe 33mpg. 28mpg probably during the winter I where I run
a defrost I all the time, and 38mpg during spring where I run windows down mostly.
I NEVER go on trips less than 10 miles, I don't sit in any notable traffic, and I don't exceed 65mph. Probably 65% of my miles are 50mph back roads car loves that.

 

[Ma v e n]

Mine with automatic gets miserable mpg until the transmission gets warm enough (2-3 miles) to lock the torque converter. Then it gets unimpressive mpg for another 5+ miles, after that everything is hot and ready to rack up some bigger than EPA numbers. Switching to 0W20 oil didn't change that, it just isn't ready to go over 40mpg at 60mph until it's warmed up for a few minutes.

Also, I suspect but don't know that the Kinergy tires on most of the RSs have more rolling resistance than the Assurance tires on the non-RS. (My 2017 RS has the Assurance so I can't compare directly)

18 Redline with 18s, Michelin MXM4s I think.
I run 5w30 in mine.