You need to compare gross weight (not load weight) and even that doesn’t mean anything. I bet there aren’t 3 of us in this thread that have the same spec trucks. That said, I’m close to @God prefers Diesels. All my loads are maxed out at 79,500 to 80,000 pounds (bulk tank). I average 5.9 mpg in an ungoverned truck (cruise usually set at 72-75).
It varies. I pull dry van and reefer-most of my loads can be between 35 to 46K gross(not including tractor fuel,DEF and reefer fuel). Lately I’ve been averaging 8.5 mpg with Walmart dedicated reefer loads(about 37K gross) and 8.3 with dry van (41 to 43K). But that’s on summer blend fuel. In the winter it will drop into the low to mid 7s(typically 7.3 mpg).