I have an 06 5.9 ccsb with a 2in lift everything else is bone stock (never tuned, stock transmission, pre def, stock exhaust). I’m only getting around 14mpg highway and 11-13 combined. Is this normal? I thought the 5.9 was supposed to get a lot better mpg. Oil change, fuel filter, air filter not too long ago any help or advice is appreciated thanks in advance!