I see a lot of forum members running 275/55 and 285/55 AT tires on 20" rims (w/Johnson Rods). Assuming your spare is matched in size and tire, can you fit that spare underneath the car in the OE location? Would appreciate pictures and comments!!

 

Yes, I have a Yokohama 275/55/20 AT tire as a spare, fits fine. I don't think you could fit any tire much larger than that though. I had measured the area surrounding my spare when someone else asked this question. It could possibly be done but you'd have to bend or cut some heat shields. The 285/55/20 tire would be 11 mm larger in diameter than the 275 (0.55 times two times 285 less 275). That's about 0.4 inches and I think my 275 is too tight to add another 0.4 inches in diameter.
Another consideration is how the tire fits on the front of the vehicle. There's a tight clearance between the tire itself and the upper suspension member (I don't recall the exact name of that suspension member but I've posted it elsewhere). That clearance would be used up by the extra width of the 285 tire vs the 275 tire. But it will depend on the exact brand and model of the tire as some tires of the same size could have a different section width than another of the same size. Search for my older post on this subject. I'm not that good at finding it and pointing you towards it.

 

To compare tire sizes, wheel diameters/widths/offsets, and clearances go to https://www.wheel-size.com/calc/.

 

Just beware, you can calculate the diameter of the tire based upon the size, but each brand/model of tire can still differ a little bit from this.

 

Excellent point. I prefer the appearance of the wider tread and overall width of the Nitto Terra Grappler G2, which is half an inch wider than the same size Falken Wildpeak A/T3W (for example). Half an inch in tread width won't affect clearance in the rear, but in the front it could mean the difference between rubbing or not rubbing when turning. If you're like me and like to have your tires fill the wheel wells, it may come down to mounting a single tire in the front to verify clearance when turning.

 

It isn't just the clearance between the tire and the inner fender. There is a low clearance point between the inner face of the tire and a front suspension component. You should crawl under and look with the tire and wheel on the vehicle.

 

The low clearance point is between the tire and the knuckle.
This is the knuckle.