Hello everyone,

My 2018 STX 4x4 use the following tire size 275/55/20. I want to move up in size to the following 295/65/20. Would I be loosing a lot if I moved up in size and can this size be placed as an extra under the truck.

 

You can use this calculator to see how much larger the tire is.
https://www.wheel-size.com/calc/

Depending on the gear ratio, engine and your driving conditions (mountains and/or high elevation vs flat land) it could be a huge hit or just a substantial hit. You are in the 10% larger range which could hit the fuel bill pretty hard.
If you have a 2.7 ecoboost, the boost will make it drive just as quick as before, however the boost amount will put you at the gas pump more often. If you have a 5.0, it will become sluggish regardless what gear ratio you have. If you drive a 5.0 in Denver, it will be a pig.

I don't know what you want to know though. "Would I be losing a lot" doesn't mean much. You'll be losing a lot of money by buying tires. That probably isn't want you want to know though

 

The spacing between the rear frame rails is 828 mm [32.6"]. A 295/65R20 tire has a calculated radius of 892 mm [35.1"]. It will not fit under the truck in the OE spare tire position.

 

You'll find plenty about what you'll lose when adding larger tires on the internet. What do you think will be affected with the larger tires?
As for the spare, it will be a very tight fit, that will likely not sit properly in the space available. I have a 285/75R18 Tire (34.8" diameter) , that is also worn, mounted in the space. It sits as flush as I want it, but it is too large to fit up in the frame as intended.