I can't really speak for a new hemi, but on a 2012 cummins my friend has was getting around 15 on the highway when new. He has since upgraded it from, turbo, head, exhaust, programmer and removed the emissions stuff. It's now making over 500hp/1000trq to the wheels and gets 16.5-17. That's with 35" tires and a heavy foot... he drives his stuff with the pedal down a lot He already windowed the original block with a broken rod from how hard he drives it.