Ecky,
I'm going to disagree with some of the things you said - but before I do, allow me to state that I just don't have the data to back up everything I am about to write. I'll present what data does exist, but to my knowledge, there's a lot of data that doesn't exist. That means we are all working off our own ideas about how tires work.
1) Wider tires do have a larger footprint - all other things being equal - except not all things are equal.
When we talk about wider tires AND having the same load carrying capacity, it means the aspect ratio is lower AND the overall tire diameter is smaller.
Let's take the 2 tire sizes the OP mentioned:
245/40R18 XL: Load Index: TRA = 93 OD= 25.7
265/35R18 XL: Load Index: TRA = 93 OD = 25.3
Please note: I have chosen to use the US standardizing organization's loads and dimensions (The Tire and Rim Association = TRA) That choice is strictly for convenience. Other tire standardizing organizations have slightly different takes on load carrying capacity, but each is consistent within itself - and the dimensions should be the same regardless.
Back on topic: The footprint size for the 265 will be wider and shorter, but larger - meaning it will have a lower average footprint pressure.
So how do I know this?
https://web.archive.org/web/20080817...on-tires-1.htm
Please note: This webpage is in the archive, not on the web today. If you dig beyond the surface, the conclusion is that wider tires have a lower average footprint pressure under the same conditions.
Are they close? Yeah - and it's close enough that one could argue that they could be considered the same - but we have to be careful because that could lead us to some erroneous conclusions.
And one such conclusion is that the rolling resistance would be higher for a wider tire (all other things being equal) - and that doesn't appear to be the case.
I developed a formula based on the only data I know of where the tires were the same, but the tire size was different. I talk about it here (about 2/3 the way down the page):
Barry's Tire Tech
Again, the numbers are close - and, again, close enough that they could be considered the same. But it is consistent with the idea that the smaller diameter causes the tread to deflect less and even though the tread is wider, the RR is smaller.
2) Wider tires have more grip, because they have a larger footprint.
Tires don't follow Amontons' Laws (that friction is NOT proportional to contact area). That's because the tread rubber penetrates the surface texture and adds additional grip.
That's why racing tires are wider. Please note: Many racing organizations dictate the tire size, so pointing out that - say - Formula 1 doesn't use the widest tires available, doesn't support the counter argument.
Why do supercars have wide tires? Mostly because of the brakes. The larger diameter wheels preclude the use of narrow, tall tires.
But a side benefit is that for the same load carrying capacity, a tire with a shorter sidewall is more responsive (quicker reacting) - which is the same reason behind using low aspect ratios in racing.
And, I don't think wider tires dissipate heat better, nor wear better. I see no reason why 2 tires designed the same but of different size would have different heat dissipation or wear properties (Keeping in mind that all other things are equal)
So I'm going to stop here. Please be aware that bring this up merely as a discussion point. I wish we had data to fill in our gaps of knowledge, but we don't.