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Leveled Sierra getting 12mpg!!!

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Hello, I’ve got a 2008 GMC Sierra SLT with z71 package. I’ve had it stock until this year (2020). Recently put a level it in and had the truck aligned and it ran great. Went to put new tires and rims which are 33x12.5 Toyo open country AT2 extremes on 18x9 fuel assault wheels. The truck obviously had a decrease in power and mpg but recently it has gotten significantly worse.


On HWY I can get 18-16 but recently in the city I am getting 11mpg!!! I live in a city with a lot of hills and windy driving so I can attribute that to some of it but it seems low.

I’ve looked around and some guys suggest checking air sensors and tune ups, but I am wondering at what point is it a bigger issue? Or is it the reality of a lifted truck with heavy tires?


any suggestions are welcome, it has not been 11mpg for long just the last month. Thanks

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  • 4 weeks later...

Don’t forget though that unless you have the tire height adjusted in the ECM your odometer is not calculating as many miles as it should. You are probably doing better than 12 but it depends on how much height difference their is between your stock and aftermarket tires. There are some calculators out there to give you a mileage adjustment. 

Edited by govener71
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  • 2 weeks later...

If your engine is a FLEX FUEL engine, you can reset the FUEL COMPOSITION with a high end scan tool. This will buy you a couple MPG, possibly. Maybe more, maybe less.

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