I have a '04 Z/71 C/C and avg. 17.5 to 17.9 mpg. Of course I don''t use the 4X4 part except once or twice per month. I live 29 miles from town so my driving is more like 85 hwy / 15 city. On the road I get closer to 20mpg. The "Sub" is probably heavier than mine. I run Royal Purple 5W30 and also a K&N air filter. Good luck!