A ranger tanks slightly better than a rogue unless you go out of your way to get tank gear, meaning Tolan's pieces and such, in which case it becomes slightly worse than a shadowknight/paladin. You'll also have a bit less hp. I'd put it like this:
If you are seriously twinked:
1-20: tanking is trivial, you'll do great
20-30: no problem
30-40: perfectly possible
40-50: doable
50+: not great
If you're well-geared:
10-20: no problem
20-30: perfectly possible
30-40: doable
40-50: not great
50+: not worth it
If your gear is kinda average:
10-20: no problem
20-30: doable
30-40: not great
40-50: not worth it
50+: don't try
If your gear is shit:
10-20: doable
20-30: not great
30-40: not worth it
40+: please don't subject others to torture
A few things to consider is that not only does ranger armor have less AC, the class also has a worse AC soft cap. You might not reach that soft cap without most of a set of Tolan's, but when you do, the difference between a ranger and a plate tank becomes bigger. The AC difference on the gear can be pretty big, too; Cobalt Helm has 25 AC while Tolan's helm has 18. That's like 40% more AC on the warrior gear, and SK/PAL armor is in the middle. This will really prevent you from tanking effectively in the higher levels where the game expects you to have a certain amount of AC. Monks don't get great AC either, but they have high innate avoidance. Rangers don't, and with the AC of a rogue and a lower cap on defensive skills compared to plate tanks, you'll really start falling behind in the last dozen levels.
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