Bad Fuel Milage

[Firefightermike]

I bought a used 2008 Silverado 1500 2x4 4.8L a few months ago. It has  slightly larger tires and a flowmaster with 2-3/4" before it and 3" after. everything else is stock. It has 113,000 miles on it. The MIL is not on, I checked anyway, no codes. started looking at all of the sensor readings while driving, they all look good except fuel pressure. Key on, not running I get about 64psi. It will hold that presser (no bleed down). After I start the engine the fuel pressure slowly drops to 42-43PSI and stays there. Idles smooth and runs great.

The dealer I bought if from was about 2 hours away, (most of it interstate). That trip and a little while after I was getting about 18MPG. Now I get really bad mileage.

14.1 @ 60MPH,

13.1 @ 65MPH

10.9 @ 70MPH

8.4 @ 75MPH

All of the above numbers were taken on flat, level ground.

 

Any ideas would be greatly appreciated.

 

[swathdiver]

Time for new cats.  Do you have access to a good scan tool like the Tech2?  You'll want to check for codes in memory and your Long Term Fuel Trims.  A bad vacuum leak at the heads/intake can cause this as will worn cats.  My mileage steadily decreased right after I bought my truck but did not throw codes until the end.  Replaced the Y-Pipe with the Magnaflow and all is well.

 

I get better mileage than that on E85!  Your fuel pressure is fine.

[Firefightermike]

Thanks for your reply,

 

I have a scan tool but its not a great one. Question, if the cat(s) were bad, wouldn't it cause the air/fuel ratio to be off?