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35 inch tires and MPGs

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I lifted my 2021 silverado RST on 35x12.50x20s with a 6 inch lift and my MPGs went to s**t... I expected a little bit of a decrease but I've seen it go down to average 9-10, when before I was getting 14-15 regularly. Now, I can only average 15 with cruise control down the highway for half an hour.


I've looked around seeing people saying to calibrate the computer for the new tires? Would a dealership do that or an outside company?

Also regearing was an option too for better MPGs? I just dont want to spend 80 bucks on gas every 5 days haha.

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You definitely lose MPG when going larger.   In my case, I moved to 35x11.5 NRG's.  I used the WAZE app to determine that at an indicated speed of 63, I'm actually going 65.  About a 3% difference.  To me, that means my MPG and distance traveled are understated by about 3%.  I have mixed highway, rural city driving and average about 17.5 MPG.  To that's about 18 MPG corrected versus the 19.5 MPG I used to average before upsizing the tires.  Seems to me I lost about 7.7% in MPG related to the change but maybe I've missed something. Idk.

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Unless you recalibrate the speedometer the computer will not read MPH right and mileage driven will be wrong.  With those being wrong MPG numbers from the DIC will be wrong.  You must 100% hand calculate with miles driven by GPS or an APP (waze etc) and then use gallons off the pump.

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I'm running 35" Nitto Terra Grapplers and my gas mileage is laughable, but hey I did it to myself. Truck sure looks good! Lol. Long story short, I'm averaging 12.3 MPG and I can get around 15 MPG if I'm cruising on the highway. 

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