Full-size Spare Tire: largest / biggest size that fits is 285/70/R17 (33's)

[Gastan]

I've been lurking often on Tacoma4g to pick up knowledge and answers that should apply to the 4Runner also. Here's helpful info about the largest spare tire that fits in the spare wheel / tire location in underbody.

285/70/R17 is the biggest spare tire size possible. This is essentially a 33" tire (32.71" to be exact).

From the owner:

I installed a full-size 285/70 r17 under the bed. It fits. The tires are wider but honestly I think it kinda looks badass with the bigger spare. It looks like it might impact my departure angle, but the tires are built to support the weight of the truck and they might even protect my trailer hitch a bit. Definitely it will get me home if I ever need it. Stock suspension and wheels, no mods to exhaust. Nitto Ridge Grapplers.​

And here's another full size spare tire used that's not as wide as the one above.

255/75/R17 (32" spare tire)

From the owner (of a TRD Off-Road):

I did a tire swap at dealership at time of purchase. They gave me $500 for OEM tires.​

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I had them install Nokian Outpost nAT 255 75 R17 onto OEM wheels AND the spare wheel. Convenience spare was 245 75 so getting the new tire to fit on spare wheel wasn't an issue.​

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This tire size is a tad larger diameter than factory tire about 1/2"..also known as a true 32" whereas factory tires 265 70 17 measures closer to 31.5".​

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Image of spare. Sits close to tow hitch receiver level and well above rear diff.​

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Is ground clearance affected..i think a little yes but it sure is nice to have a proper full size spare that I can trust vs that convenience spare crapola.​

 

[Gastan]

Added another spare tire size -- 255/75/R17 (32") to first post above.

 

[DustyLens]

I'd be concerned about departure angle as well. And I don't think the spare should be visible under there.
Having said that... I was behind a 4Runner the other day which had its spare tire mounted to an after-market swing-out system -- bumper-mounted I expect (after-marker bumper as well?). Which makes a lot of sense if you ever have a flat out in the wilds and can't access the underthing.