I was playing around with a gear ratio calculator but I don't how to interpret any of the numbers. I assume my towing capacity is reduced but was looking for a push in the right direction to try and nail down some numbers. Buying a travel trailer and have ruled out trading for a Tundra and I want to squeeze every bit of our 5000 lb towing capacity. Thinking it would be wise to get a trailer with a gross weight under 4000lbs so I can load up with gear. Thanks in advance.

As long as your tires are rated correctly then you will have no reduction in towing capacity, your 4Runner will just shift more frequently. Tire weight is unsprung so it does not calculate into the GVWR. The larger diameter means you have less wheel torque but your 4Runner will compensate by shifting even more.

You need to subtract the weight of all the other crap you have added though like that front bumper, skids, sliders, etc because that will reduce your towing limit.

With that said if you want to pull 5K more than a few times a year then buy a pickup truck. Our 4Runners will pull 5K but it's not pleasant, especially in the mountains. You will also get 10-12 mpg with that load. You will also need a brake controller. I pull around 4K a half dozen times a year so the 4Runner is fine. If I pulled any more than that and I would buy a Tundra or some other 1/2 ton V8.

I had a 2012 Tundra before I traded for the 4runner. Towed my buddies race car to Florida and back with two spare engines and transmissions in the bed multiple times. There were times I forgot I was towing a trailer. 5.7L and 6 speed for the win.

The heaviest I tow with the 4runner is a four wheeler, dirt bike, and fire wood all at the same time. I can always tell I'm towing something.

That said, you should be fine with your plan of a 4k trailer. Just expect some down shifting and gas stations in your future.