www.fueleconomy.gov or Nissan's factory website.
I used to get around 16 in the city and 20-21 highway. I added a new intake and exhaust so i get more now but haven't measured exactly how much more.
your MPG can be effected if you disconect your battery. this causes your ecu to be re-set and the engine computer goes into the RE-LEARNING PROCEDURE. you must start the truck and let it idle and all fluids at operating temperature. then you have to drive the truck while its re-learning the air/fuel mixture while you drive the truck. you must not go over 1500 rpm's. keep rmp's from 1000 to 1500 on your way to an open highway . on highway very carefully bring truck up to 60 mph for 15 minutes (flat highway) this will finish your re-learning procedure. my S auto gives me close to 400 miles per tank on highway at 65 to 70 mpg with a/c on.
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