My Impala is a 2014. Probably the same engine, whatever the largest is. I get lost when it's not a 350, 400, or something like that. It gets upper 20's around town and on long trips at 80+ I averaged 33-34 MPG. Like you wrote the real average is closer to 30 highway. I use nothing except the lowest priced regular I see. I don't worry about it as long as it's better than my ex-truck at around 15-18 MPG. For years I'd tow an RV and with anything I drove and got 9 MPG. Gas was cheaper and I made more money.