Figured this probably didn't qualify for the Tire and Lift sticky as I am not wanting to do a lift. I'm just looking to see if I can put wider tires on to make it a little more flush with the fender flares.
My 4Runner still has the stock tire size P265/65R17 with stock rims. I am curious to see if I can safely put on wider tires with no other modifications. Something along the lines of P315/55R17. Can it use the same stock rims or would those have to be changed too?
After significant research on wheel fitment, I can tell you that it likely won't matter what lift you attain or chose to not attain with any given tire size. As long as you don't plan on turning the steering wheel or going over bumps, it'll never rub... uh, I mean, well, the suspension is going to have to articulate and you're going to have turn the wheels... so a heat gun and a die grinder will be your friend to make it work. Regardless of lift, the suspension still cycles within the same boundary (unless you move the bump stops)... so adding height to fix rubbing only alleviates rubbing at that height.
More to the point of your question, -12 offset on a 9" wide rim will put the rim face at approximately flush with the crest of the fender line. 0 offset on a 9" rim will put the face of the rim sunk inside the fender 0.5".
Mileage will vary as to how much clearance that affords you to the upper control arm on the inboard side of the wheel. That will depend on the section width of the tire we're talking about (and a bit dependent on tread width).
One concern with anything over 285 wide on a 9" wide wheel at -12 offset is that the tread will likely poke out beyond the fender which may or may not be legal in some states/provinces. So a 0 offset would give you 12mm more width to tuck in... but 12mm less space to the UCA.
I have 265/70R17 grabber AT2's on stock rims (+30 offset)... and I think there's about 8mm to the upper control arm. I can moreless get a finger between the two.
A few things to consider with a much larger tire is tire+wheel weight. How strong are your shocks? How do you drive the truck? I assume you have stock/OEM style suspension (since you don't have a lift). Any wheel worth owning is likely going to be atleast 25-30lbs in 17" diameter, 30-35lbs in 18", and on up from there. Any assembly over (this is guessing) ~80-90lbs is likely to cause some handling deficiencies if your suspension is tired/worn out. Your tires will bounce over obstacles under duress when they otherwise would have traction because the shock/spring combo was not designed to control the extra weight of tire installed. Anything about 100lbs could actually break the shock... someone posted a picture of a Tundra on 35" tires that snapped the lower eye of the stock shock off when he hit a speed bump.
You'll want to (ideally) stay within or very close to the approved rim width of any tire. Going outside of that will produce some weird handling in some situations. People do it though, and few have problems but beware...
Lastly, as far as I could turn up 315/55R17 isn't a production tire size. Closest thing I could find that was crazy wide and short was AMP Terrain Gripper 305/60R18, which is a 32.3" tall tire. If you want an off-road style tire, you'll be hard pressed to stay under 31.5" tires (285/60R18, which is reasonably common) and retain a profile greater than 285. If you want an onroad tire, you may be in luck to find shorter and wider tires, but your rim diameter and pricing may get crazy.
I have some measurements of the wells somewhere, I can find them if you wish.
