My $0.02 is what ultraclyde said. Run the largest tire that comfortably fits. It's like your truck, you want a tire that stuffs in the wheel well without rubbing the frame or fender. So it is with a MTB, a tire that fits without rubbing a stay when you consider it packing with mud or in the case of touring bike especially if the rim is knocked a little out of true. I think you have a good handle on what you're after.I should've been more specific. The tires themselves work well. However, I have little-to-no clearance under the fork for the front tire or from the front derailleur to the rear tire. After a quick google, the max recommended tire size for the Trek Mamba out of the box is 2.1. Perhaps if I went to single front chainring, then I'd gain the necessary clearance to keep a larger tire.
What do you think? If I just have to get used to debris (leaves and such) getting stuck on the FD hanger, then I'll adjust my expectations. I've done WAY more road riding than mtb, so I'm very open minded.
My experience is usually the max size means just that, the largest tire that fits normally without being so tight as to not function. Also remember that the diameter and casing width isn't anything like standardized, so one brand and model 29x2.1 might be the same as another 2.0 or 2.2, so there's some trial and error here.
I usually end up with a tire that is a little smaller than a specified max, e.g. a 2.1 when the shop said it *should* fit 2.2. Most of the time my frames end up with two bare spots on either side of the tire due to trying to figure out what tire works best.
And yup, you can often squeeze in a bit more tire by eliminating the front derailleur. I run nothing but single front rings. But I also run single rear cogs so clearance is purely driven by my frame.