Wanted to post my research and share my decision on selecting larger tires on a Factory '21 AT4, 20" Wheels.
There are a lot of threads on this topic, all with great content, but there are some conflicting posts on size choice/rubbing/mpg impact that can make the decision difficult. As others have noted not all tires measure the same height/width/weight, so a one-size fits all approach likely cannot be taken.
**In case you don't want to read through all the details below, I went with BFG KO2 in a 275/65R20. No rubbing, plenty of UCA clearance, ~1MPG loss, speedometer is off between .5 to 1MPG depending on speed. ~1MPG loss does not reflect accounting for speedo variances. I will need to update this post with a manual calculation. I do think you could fit a slightly larger tire, depending on manufacturer choice (see below variations of sizing).
Tire Size:
Based on reading through other posts I narrowed my tire size to three options:
1. 275/65R20
2. 285/65R20
3. 295/60R20
I personally didn't want rubbing issues after tire install. Minor trimming (IE: mud flap trimming) wouldn't be a big deal, but having owned a truck that had rubbing issues, it gets old quick and I wanted to avoid modifications if possible.
Pros/Cons of each tire size I contemplated:
275/65 - great tire size as you get 34" in height, but can appear to have a narrow stance depending on tire choice. Stock tire (Bridgestone Dueler A/T 275/60/20), width would remain the same as stock. Originally ranked 2nd of the 3 options.
285/65 - best of both worlds, gains in height and width. Ranked 1st of 3 options. Big concern around rubbing issues, especially at full lock on an incline. Likely could avoid by adding a levelling kit. Conflicting results on CV angles and longer-term impact of adding a level kit.
295/60 - prefer the stance of a 295, but didn't have a high level of confidence there would be clearance from the UCA. Rubbing concerns similar to 285/65.
Tire Brand:
1. Falken Wildpeak AT3W
275/65 - 34.1" X 10.6", 66.1lbs
285/65 - 34.6" X 11.2", 67.7lbs
295/60 - 33.9" X 11.6", 69.4lbs
2. Toyo AT3
275/65 - 34.1" X 11", 54lbs
285/65 - Not Offered
295/60 - 33.9" X 11.8", 60lbs
3. BFG KO2
275/65 - 34" X 11", 57.3lbs
285/65 - 34.5" X 11.5", 60.3lbs
295/60 - 33.9" X 11.8", 68lbs
**4. After decision came across Mickey Thompson Baja Boss A/T (some great reviews coming out on this tire).
Pros/Cons of each tire size I contemplated:
Falken - awesome tire with some fantastic reviews. Major consideration was the weight.
Example: 275/65/20 weighs 66.1lbs
Toyo AT3 = 54lbs
BFG KO2 = 57.3lbs
Toyo AT3 - I've had several sets of Toyo AT2's. Really like the upgrades made to the AT3, but wanted a slightly more aggressive sideway. I also found it very difficult to find inventory at the moment.
BFG KO2 - tried and true, some question around if newer technology is surpassing. Obviously not enough of a concern as this was the tire of choice.