I've put about 1400 miles on my new 2021 GMC Sierra 1500 Duramax Diesel with the goodyear wrangler tires and 18" wheels. Most of my daily commute is city driving where average speed gets to about 40-45 MPH. My average MPG has been around 17 MPG which is about 23% below the rating of 22 MPG. I make a conscious effort to be hyper aware of driving habits such as keeping RPM's low (not gunning it between gears or when stop lights turn green). My daily commute obviously isn't a flat perfect road, but I'm a bit skeptical that i'm 23% below the rating (i'd be much happier with a less or equal 10% variance). I live in California (northern), so environmental standards are pretty much overkill compared to other states, and making sure to read the label at the diesel gas pump.
What other factors should I consider to improve MPG?