2003 Gmc Sierra 2500hd 6.0L 9.6 mpg slight stutter at idle

[Marshall Pay]

To start I know that the 6.0 is a pig on fuel. On average I get about 9.6 mpg on highway not towing. Is this normal? Also should mention I have a new belt and pully system, fuel filter, plugs and wires, exhaust is sealed with new gaskets, new maf sensor, intake gaskets were replaced and the throttle body is clean, new air filter, all the 02 sensors are working correctly. No engine lights on. Previously I had a rear knock sensor code, replaced both knock sensors. Code wouldnt go away so I ended up splicing the rear knock sensor to read off the front sensor. Code cleared and economy definitely got better. Am I missing something? Alot of forums I see people are getting 12-15 mpg on average. I'm not driving like a maniac. I do routine oil changes.

Edited December 20, 2023 by Marshall Pay

[CamGTP]

That's a tad low if you ask me. We had a shop truck that got 10-11 just about all the time but if you did straight freeway it could get a little better.

 

Bigger than factory tires and doing anything over like 65mph it would drop the fuel mileage.

 

If your fuel trims are within like 5% of zero, then you know that that stuff all checks out so it's not using extra fuel from a lean condition.

[Marshall Pay]

8 minutes ago, CamGTP said:

That's a tad low if you ask me. We had a shop truck that got 10-11 just about all the time but if you did straight freeway it could get a little better.

 

Bigger than factory tires and doing anything over like 65mph it would drop the fuel mileage.

 

If your fuel trims are within like 5% of zero, then you know that that stuff all checks out so it's not using extra fuel from a lean condition.

I have factory spec tires, I live Canada so the highways 110km/h which is just over 65mph. Still I feel like it's drinking more than it should. I'll check out my fuel trims again but last time I checked they were good. I think the best millage I've had is 9.8 around a year ago but the average has not changed since I hit 9.6 about 9 months back regardless on how I drive. Same with or without ac it stays the same