5.7 Poor fuel mileage

[NDFARMER95]

Bought a 96 2500 EC/LB while back. Has the 5.7 with auto trans. Replaced intake manifold gaskets. Rolling on 265/75/16 at stock height. Temp gauge don't get over 165. Running 5w40 synthetic. Need to replace the fuel filter. But anywho, it gets like 12 mpg hand calculated. Then come to find out the speedo is 5 mph fast. So that would make my fuel mileage worse. My old 6.0 chevy got about this on 285's and 4 inch exhaust. Any ideas? It starts great every morning and runs great with 155k on it.

Before some keyboard crusader tells me its a 4x4 and it's going to get ty mileage, I have a duramax with a 6" lift and 35's also so I know what to expect, but these 5.7's can do better than 10-12.

 

[Birchyboy]

12mpg where, city or highway?

 

[df2x4]

Just a thought but where have you been filling up? I've noticed that I get considerably worse gas mileage whenever I fill up at a Casey's, Break Time etc. but when I go to larger chains like Phillips, QT or Shell/BP everything is fine. Something to do with their suppliers maybe.

 

[NDFARMER95]

12 mpg average. Mostly highway. Fill up from the tanks at the farm.

 

[96Z71ECSB]

195 degree thermostat will help.
Your truck temp should be running between 190 to 210 degrees stock.
Any lower temps, engine fuel efficiency will suffer.
It won't be a dramatic jump in fuel mileage, but it will help.

Other things to try are the basics.
New cap, rotor, wires, plugs, and filters. But you may have already done that.

 

[NDFARMER95]

Changed out the thermostat. We'll see how that goes I guess.

 

[magimerlin]

O2 sensor....it doesn't have to throw a code to kill your milage.....

 

[NDFARMER95]

Today it started surging at idle while just sitting there. Fan motor slowly runs with the switch off. This thing is full of gremlins.