Can I use 285 65 20

What is the stock size? You can compare tires with this handy tire size calculator ---- https://tiresize.com/calculator/

What is a stock size if you mean what is on now it's 17

Stock is 16". To get even close to your original gear ratios, use the calculator FoR suggested. You will have to go to an extremely low profile tire, causing poor ride, and stupid looking

https://www.google.com/search?q=2005+GMC+Yukon+XL+1500+SLT+&ie=utf-8&oe=utf-8 == Wheel size: 16″ diameter,

Your speedometer will read 65 mph at a true 73.5mph. Changing wheel/tire sizes is not so bad, but you are decreasing your gear ratios all the way through the spectrum it will tax your transmission. You NEED those lower gears to get going

Big wheels and tires are heavy and will increase stopping distances considerably. Heavier wheels and tires will also reduce ride quality as well. You could use the extra room that 20" wheels provide to install some monster Brembo brakes though.

FordNut is right. Big tires will reduce your towing and hauling capacity especially in the hills. It will also reduce acceleration and increase passing distances.

One thing I can say with absolute certainty is you can trust tennisshoes advice.

I run 285's on my Ford F-150 which increases my ground clearance 1/2" and improves handling and off road capability quite a bit over the stock 265's.

As long as you put tires on that have the same outside diameter of the standard tires, the speedo will read correctly. You can put Low Profile tires on bigger rims, as long as the tread outside diameter is the same as the stock tire.