I expected a little bit of a drop, but I lost 10-12mpg.
crazy!
…but it looks good. 2018 xl 4x4 Regular cab short bed, bilstein 5200 front and rear, air ram delete, rtx off road rims ( slate w/machine face) +25mm with 285/75/17 big ko2.

 

You LOST 10-12 MPG, or your MPG went down to 10-12? If you lost 10+ MPG, something isn't right. I put a 2" level with 35" tires on my truck and only lost roughly 2 maybe 3 MPG.

 

i have to do more driving, but yeah, i was getting 30+ mpg on the highway and i'm 20-ish mpg now. it was a pretty huge drop. i used to set my cruise to 58mph in eco and get 36mpg one way on my weekly trip to see my dogs ( and ex ) and got about 30mpg on the way home. 80 miles each direction in different altitudes. i had to take a trip for work today to pick up some parts a few towns away and was pretty disappointed. my next trip tp see my doggos is the benchmark for me, but it's not looking good for the ol' fuel economy
i'm going to try and chalk the tires for air pressure and see how far off it is. they are set to 38psi which doesn't seem low, but may they need a few more pounds. idk.

 

How are you calculating mpg, and are you taking into account the larger tire diameter when calculating the new figure?

 

No way this is accurate.

 

Your speedometer/odometer has not been calibrated for the new tires - you have not lost 10-12mpg.
The taller tires are turning few revolutions per mile thus the miles are reading lower than with factory setup.

Get a tuner or use forscan to correct the speedometer/odometer and most of your 'loss' will come back.

You can check the percentage of loss by using a gps app and seeing how far off your speed is from gps to truck.
Without knowing the factory tire size, I'd say you're at least 6% off.

 

I lost a few MPG, but didn't buy a Prius. But it was totally worth it, and I expected it.

 

no way on Gods green earth you lost 10-12 unless you poked a hole in your gas tank by accident doing so.

Ive had a level, air dam removed, LT34 inch tires (weighed 65lbs each), and so forth....never lost more then a 1-2 MPG drop...i would suggest doing hand calculations. At best, my 2018 2.7 stock would get 26ish mpg at 60 mph cruising down the highway on cruise control.

I would do some hand calculations, but also my hand calculations have been pretty much spot on with the trucks computer

 

I don't know how accurate it is with the OP, but have heard of mileage taking a decent dip with the larger tires. Mine is on 33's and I'm getting 15 MPG, but I've heard of stock 5.0L's getting 20-24. Not as big a dip, but still substantial with what gas costs.

 

This is the problem. Also, there is a method to put your truck into engineering mode and reset your AFE. Just Google it and you should find step by step instructions on how to do it. After doing both of these, your MPG display should be more accurate.
If you aren't running an aftermarket tune or don't have a Windows base laptop for Forscan, I believe there is a Hypertech device that can correct your speedometer. Or, check around some local Jeep or lift shops. They might have some other methods for correcting the speedometer.

Also, what engine do you have?