I have a few observations - but no solution.

Are your tires properly inflated? Cooler weather causes the tires to loose pressure (affecting mileage). I don't always buy the "winter gas" explanlation - as the a/c system is not used in ther cooler weather (other than defrost), which lessens the load on the engine. Did you switch gas to a different brand?

What is the window sticker MPG estimates? I checked the government fuel economy ratings and there are so many different ones as your vehicle has so many variations and configurations.

Are you driving in AWD or 2-wheel drive mode?

 

18 -19 is a pretty good MPG average for the new Silverado. I am averaging 18.3 in overall driving and am very happy with that.

 

And I would add checking it via pencil and paper, (use a program like Gas Buddy or Fuelly) I do for every tank, so you can get a feel for seasonal gas and weather trends.

The other thought is they "fixed" your transmission to have better drive-ability, at the expense of a drop of economy; sounds like a VW diesel?

 

There is going to be little to no difference when changes are made to the transmission shifting and lock up points. We have made changes with HP Tuners for years in my shop with no difference in the over all fuel economy. A 5.3L engine with a GM set up or one with my changes gets the same mpg and we have over hauled hundreds of units and have never seen or heard from customers of a drastic change in mpg.

 

Thank you all for your replies.

I am meticulous about my tire pressures. In fact, I have them all set at 35-36 pounds, a bit higher than the plate.

The sticker on my truck is 23 highway. I don't recall the "city" rate. The bulk of my driving is 50mph+.

My truck is 4x4, but I have been driving in 2WD mode since I have had it. That is why I was so happy to be getting 21. I figured the 23 was a base 2WD model.

I have a complete log of my mileage by tankful. I calculate it manually and compare to the last reading on the DIC. With few exceptions, they are within about 0.5 mpg before as well as after this mess started.

I will be going in for my next oil change in about 2000 miles. I am going to suggest the transmission "fix" suggest by Ed753. I had not considered that something may have changed by simply checking for error codes. We'll see how that goes.

Thanks again.

 

Winter grade fuel and colder temps will make a difference. Wait until January comes and see what you get.

Here is a site that names 9 reason's why your winter MPG's suck.
http://www.metrompg.com/posts/winter-mpg.htm

 

I guess what bothers me most is that I was getting better mileage before this service and I want it back. Something changed during that service and it seems to me that all the explanations (reset Instant Econ page, winter gas, etc) are not making sense. Especially since this started in mid-August with its warm weather and "summer gas". I'll give that I may see a drop with the "winter gas" and/or colder weather. But why was there a drop in August/September?

I even remember my salesman bragging that another Silverado customer was getting 21-22 as well. So this kind of mileage is not unheard of.

Thanks for all the feedback.

 

Every car or truck I have owned loses 1-2 mpg during October to around April months with the colder temps and Winter grade fuel. The mileage always shoots back up to normal thereafter.