That is surprisingly low for a newer truck, IMO. My 2005 GMC regularly gets 15+ with remote start idle time, mostly city driving.
When the 5.7 Hemi first came out, one of the jokes was that it made small block HP, but got big block MPG
. Admittedly, I do not have any seat time in a newer 5.7 truck, only newer EcoDiesels. We were typically seeing 26-29 MPG empty and 18-20 MPG loaded down with those trucks, on the highway.
FWIW, depending on the year of the vehicle, you can trust the cluster MPG calculation. One of the manufacturers was in trouble some years ago for inaccuracy related to the vehicle-displayed MPG calculation, and they're required to maintain a certain level of accuracy now (with OEM components and tuning, anyway).