Correct in thinking putting wider tires than rim was designed for will help.
The width helps on some terrain for sure.
Having a sidewall flex helps too.

But on these 20s, its hard to get much for tires. If you had taller sidewalls, that would help.
Tread pattern is important, as we all know too.

I have had 2 4runners with the 20" limited wheels
I have had a Tacoma with the 20" limited wheels
I have a Tundra with the 20" limited wheels now
I have a 4runner with the 20" limited wheels now

I have had (2) sets of 285/55-20 BFGoodrich
I have had (4) sets of 305/50-20 General AT2 and ATX
I have (1) set of 305/50-20 General ATX on my current 4runner
I have (1) set of 275/55-20 General ATX on my current Tundra

It seems everyone is scared of putting wide tires on the stock 7" wheels, its seriously no problem.... I have worn 2 sets of 305/50-20s out on these wheels and never had a problem. I got 90,000 miles on my Tacoma with this tire and wheel combo and then replaced the tires with another identical set and eventually sold it. These wheels will handle tires, just put them on and go riding.

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An old thread, but I just tried swapping out the stock Geolandar's for 275/55/20 Michelin's on my Limited at my local Discount Tire. The computer system flagged the larger size as not compatible and they would not do it. Looks like I'll be looking for new wheels. I guess it really depends on where you are whether this is an option or not.

Unfortunately, I've run into this with them twice at 2 locations in 2 different states (you'd think I wouldn't have tried a second time .. but I'm not too bright apparently). They have good prices, but not good enough to make me NOT get what I want for some bizarre unwarranted "legal risk mitigation" they have.

So ... the solution is simple. Don't do business with them. I've had Kauffman/Mavis put on 2 sets of Michelins in that size (LTX2 and the renamed to "Defender" on 2nd set) over past 6+ years. I just had Firestone put on some Destination LE3s in that size couple months ago. Neither said a thing.

LE3s seem pretty good. I've had LE2s on my 4th Gen. before and they were pretty good then too. There were some complaints about siping performance in wet weather on LE2s after a bit of wear ... and that has been solved with full depth siping in LE3s. Time will tell ... but so far so good and price was good.

PS - Still driving the 2014 Limited ... 180,000miles and running like new.

Those wheels look sick on the tundra. Any spacers on the tundra? Rubbing? You think 275/60/20 will rub on stock 1st gen tundra?

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Go somewhere else..... Discount tire sold me almost all of my tires but they wouldnt mount them, so i took them to other shops. They have a STRICT policy on following OEM tire specs on a vehicle, they WILL NOT go outside their margins for "liability" sake.
Fugg em go somewhere else if you want the 275/55-20 go buy them somewhere else, or buy from them and take them to another shop to install.

I did the math on the wheels, the OEM 20" limited wheels sit out a little further than the OEM 17" wheels, only like 1/2" or so. This allowed me to run a taller and wider tire without rubbing the A-arm.

Depending on your wheel and offset, yes you are most likely going to rub with 275/60-20.
Stock wheels, you are going to rub the upper A-arm, inner fender when turning, and likley the frame.
If you get aftermarket wheels with more offset, you might clear the A-arm and frame but you are going to hit the inner fender and pinch weld worse.

I have run 305/50-20 on my Tundra, with the 20" limited wheels, and there was substantial rubbing on the upper A-arm and substantial rubbing on the mud flap and inner fender, so much so it limited my turning radius.
I then put 275/55-20 on my Tundra, with the 20" limited wheels, and there is no rubbing on the upper A-arm but there is only a business card between the upper A-arm and the tire, and i only rub on the inner fender in off camber full lock turns like hitting a tall lip/curb pulling into a gas station with an incline.

275/55-20 - 31.9 x 10.8
305/50-20 - 32.0 x 12.0
275/60-20 - 33.0 x 10.8

Going to an even larger diameter, is going to push the tire closer to the fender even more when you turn, and closer to the frame when you turn. I would speculate its going to take about a 2.5" lift, new upper A-arms to correct caster, longer sway bar links, no mud flap, and pinch weld mod to clear a 275/60-20 assuming you stay with a narrower OEM style wheel like me. If you go out and get a 20x9" with 0-offset, you are going to be cutting stuff on the fender to keep them from rubbing when you turn.

Do you just want the looks of a 275? I have a car that has 9.5 inch rims and the tire on it is 275. Because that fits it properly for sidewall to be effective etc.
And really, are you going to really get more grip with a 275 vs a 255 or 265 width tire.
I think it may all be a visual thing people want when they mount a tire too big for the rim..

Sure, part of it is for the look of the 275. The 245 are so thin. I'm currently considering the 255/55/20 size.

One thing to look at is, not all manufctures measure their tires from same area.
Years ago, my 315/35/17 tires on my Corvette needed replacing. Sumitomo, Good Year, BFG, Nitto and Michelin were all different when mounted on the rims.
All had same size, but there was a visual difference.
I saw some Good Years on a 4runner limited that were the 255s. Compared to OEM tires, the Good Years looked beefier, probably were. But, the treads were far more aggressive. Not sure how much the fenders would need to be massaged, but hey looked good.
I personally put Spidertraxx on mine, and it puts the tires out a little further to visually look a little better than OEM.

My Tundra has 275/55-20 General AT/X all terrain tires
My friend has 275/55-20 BF Goodrich All Terrain TA/KO tires on his chevrolet 4wd

Both of them side by side in a parking lot look totally different, mine look wide, beefy, his look thin and anemic. Looking at his from behind like at a stop light, you would think his tires were a 255 wide, in comparison to mine they look like more than an inch narrower. Its a BIG difference visually between the two even though they are the same size on the sidewall and in the same category of all terrain tire.