Large decrease in MPG

[fullsendautodetail]

So I have a 2018 Silverado 5.3 Z71 LT and I've tried looking up threads for this topic before but for some reason I haven't had any luck.  Ive had this truck for a year now and I had a level lift installed and my MPG stayed around 18-22 MPG and I could get 325-400 miles out of a full tank of fuel.  I recently installed a 3.5 Rough Country suspension lift and I installed 20" wheels with 33" tires.  I knew going into this modification that I would see a slight decrease in MPG but immediately after the install I dropped my MPG to 7.5-9 MPG and I cannot get it to increase AT ALL.  I used to work at a GM dealership and spoke to a couple of old co-workers about this issue and for some reason we cannot pinpoint the issue.  I know that increasing the ride height and the different wheel/tire package would create a difference but I didn't know that it would be this bad.  I'm considering a performance chip and a CAI to try and increase my MPG, and its not a bad batch of gasoline and I've changed my driving habits and kept my foot off the throttle a little bit...if anyone has any info on this it would be greatly appreciated, thanks a lot!!

[CombsL83]

3.43 gears?

[fullsendautodetail]

Yes 3.43 gears 

 

[CombsL83]

Wouldn't think you'd drop the much either. What type of 33s do you have? All terrain? Mud terrain? PSI in your new tires could be an issue

[Tx_operator87]

On my 2015 Silverado 5.3 when I leveled it I kept a consistent 17-18mpg, mostly city driving. I had a cai and the diablosport tuner on it, when I put the Toyo open country at2's on it (33x12.5) I didn't recalibrate the speedometer right away and went down to 13-14mpg, hand calc. And when i recalibrated the speedo I was staying around 15-16mpg with consistent hand calculations

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[fullsendautodetail]

I have 33” mud terrain tires, granted I live a mile from work and I typically put 15-25 miles a week on the truck so it doesn’t see a lot of highway use. I’m taking a trip this weekend about 3 hours away so I’m goin to use some fuel stabilizer and open it up. I’m not 100% sure what psi to run the tires but they were sent to me with 32 psi..the tires read to run them at 35 psi