I have never gotten better than 28 MPG highway since day 1, 2011 LTZ RS. I've brought it in more than once while it was under warranty. I waited about 10K miles figuring the engine needed to break in but it never got better. Dealer blamed it being the LTZ with 18" wheels causing more rolling resistance. I can believe that affecting MPG to a small extent, but not a 20% loss in MPG over a slightly slimmer tire. I was getting better MPG in my C6, and that has some very wide tires.
I'll be trying some Tehran. Having other issues right now though.