I traded a perfectly good 04 Silverado LT extended cab 5.3 engine with the heavy duty tow package (3.73 rear end)
I was getting average mileage of 17.5 to 18.6 This included intown driving and some road (interstate miles) I needed more room so I foolishly thought with the AFM on the 07's I would come close if not exceed my old mileage. I bought an 07 Silverado crewcab LTZ with the 5.3 AFM 3.73. I am lucky if I get 13 miles per gallon! I'm driving easier on the same roads and routes I did before. I'm lucky if I get 15 on the road. What could be wrong??????

 

I just got back today from a trip that was combinedinterstate/2 lane hwyback road. My 07 Silverado5.3/3.73`s with AFM got 18.6. It has averaged 16-17 mixed has I liveslightly out of town. For me this is an improvement has I sold an 91 GMC that was a good truck,[5.7/3.42`s], but did not get quite that fuel mileage.

 

The first 1000 miles on my 07 with AFM is 18.3 mpg.

 

Remember to check your fuel cap.Mine has gotten loose already once and that affects your mileage.

 

Something else to think about is that while it is engaged only four of the cylinders are getting fuel so you save gas. But the other four are still their moving to, because of the engine still having to move the other four cylinders it will rob power from the engine to keep them going. So your gas mileage will suffer from that view point. The question is do the four running cylinders have enough power to keep all eight moving without using the same amount of gas in the long run.

 

Ok, I will bite. What does a loose gas cap do to affect mileage?

 

Since the valves are deactivated during 4-cylinder operation the dead cylinders produce virtually no parasitic power loss and you can easily see (on the DIC display) the enhanced MPG as the engine transitions from 8-cylinder to 4-cylinder mode.

A loose fuel cap will not effect fuel economy or engine performance but it may trigger a "Service Engine" light on the IP.

 

Not being an engineer I do know that in theory the fuel cap will not have much effect other than allow excess air into a closed system which will as stated above throw aservice engine light.

 

Then what did you mean by this?

 

The gas cap thing affecting mileage is some distorted urban legend.