I have a 2008 GMC Sierra I bought used in May. It has 37,000 miles on it now. It has the 4.8L and the 3.23 rear. My gas milage was 18.2 when I first bought it. I was disappointed in that and now it's getting worse. 15.6 the last time I checked it. Any ideas as to what to check? Also according to the build sheet it's supposed to have the cylinder deactivation option. How can I tell if it does and how do I know if it activates? Thanks.

 

The cylinder deactivation is designed so you don't notice it. It responds in milliseconds based on throttle input and engine load to give you the best fuel economy. That being said, the computer also learns your driving habits and the only way to look at that info is with a computer scan tool with appropriate software.

 

For a heavy truck 18.2 is not bad at all. A truck is not really made to get 24 mpg's my friend. The same truck with a 6.0 yields about 12- 13 mpg's but will tow and haul almost anything. So if you can get back to 18'sh, thats good. First clean MAF sensor, TB, clean air filter, and check for lazy O2 sensors.