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Recently (started three months ago) - I have noticed a sudden loss of fuel economy in my truck. I drive 500 miles a week ALL highway. I have gone through 2 winters on it (bought with 20k miles and now has 70k miles). In september 2016 - I noticed my highway milage dropping. I used to get 21-22mpg on my 170 mile commute (all highway 70mph). It performed like this since I bought. I started seeing 17-18mpg highway, and now I get no better than 15mpg highway. My average over 50k miles driving was 20mpg My average over last 3 months and 4000 miles is 14.5mpg While on the freeway at 70mpg - it gets 14-15mpg. This is very frustrating because for past two months I have been troubleshooting with my dealership. I have extended bumper to bumper and full drivetrain warranty - none of which covers this issue!!! They can't tell me whats wrong with it and seem unable to diagnose the cause. We have tried the following: -Put different stock tires on the vehicle and drove for two weeks, no difference -Put a MAF sensor from a different (new) silverado and drove for two weeks - no difference -Tested for fuel consuption - no excessive use of oil (very little to none being burnt) -Oil changes - no difference -New stock airfilter and clean MAF sensor - no difference -I use only top tier gas and oil -No tire wear visible -Fuel trims appear to be good -No diagnostic codes showing up I know there are many variables for fuel economy, but we have look at everything we could and have ruled out most everything. The dealership is of no help and says GM has no documention on this issue elsewhere. I have talked with many auto professionals and they all say this sounds bizarre or offer up that I have a crap engine. Regardless I have full bumper to bumper/drive train warranty and its of no use!!!! Does anyone offer up what I should take for next steps? I mean, I used to own a 1971 Chevy C10 3 on the tree, that got better mpg than my 2014 Silverado. Its rather unbelievable Where do I go? Who can properly diagnose the issue? Any help appreciated! Thank you! -Darren
