For tanks, it ONLY takes into account your top Battle Rating tank, so for example if you unlock a BR 4.7 tank you may be better off sticking playing at all 3.7's until you unlock another 4.7, otherwise you'll be facing much tougher tanks in mostly 3.7's. (And you'll have stock ammo, and no repair toolkit in your 4.7)
When I see a KV2, I normally scream and run away. I have however, managed to kill one in my mighty T50 (from very close range).
I've been playing this for the last month or so and I'm quite enjoying it, apart from having to play arcade mode (the wait for the other games is too long). I started with German tanks and worked my way up to a Stug III F and Panzer IV F2 but decided to try Russian tanks after continually getting smashed by T34s, KV1's and KV2's. Now I have my KV1, it feels like I'm continually getting smashed by Panzer IV F2's and Stug III F's(<- aimed at myself).
Now I'm not expecting to be the dominant tank in any match I play, of course not, but I can't work out where I'm going wrong. I read a few pages back about the Battle Rating calculation, so looked at mine and saw that the KV-1 ZiS-5 was taking me into a higher bracket (I did have some matches against Panther and Tiger tanks) so I took it out. Now I'm not facing those tanks (phew!), but still struggling a bit.
I was going to ask for advice (any advice is still welcome of course), but I've just read a previous post by matt_fsr which has some good tips that I'll work on.
Any chance I can join the OcUK squadron?
See comments above on Battle Rating calculations. The KV1 Zis-5 is a 4.7 so will drag you up to higher battles, yet it offers little benefit over it's 3.7 sibling I find.
Soviet tanks are a mixed bag early on, some great fun, some are steaming turds - they seem to have a strong 2.7BR lineup and also around 3.3, but struggle above that until the appearance of the KV2 at 4.7, soon to be upgraded to 5.0 apparently. (45mm of armour on a T34 is great at a BR where a lot of stuff can't punch through it, but less great as your opponents improve)
At BR 5.0, stronk Russian tonk murders everything
- if you use them correctly. KV2 and SU152 both throw 152mm of HE or AP death, which apart from the occasional really unlucky bounce off sloped armour, pretty much guarantees a kill. The 3rd tank can be either a KV1, or T34, or if you're brave bring along the ZUT37 of trollness 
6.0 Soviet brings you IS-2. The Mjolnir of tanks. 'Nuff said.
To defeat tanks, you need to start learning weak spots, and where certain models store their ammo, so you're not wasting shots on just causing damage. Pz3 and Pz4's have vulnerable driver glass, so if you can stick a shell through their front where the driver sits - insta-kill.
Lots of tanks are also vulnerable to turret hits, especially ones that store ammo in the turrets. View the tank models in 'outline' mode or whatever it's called to see where shells are kept, and aim for that spot when you're taking them on.
Finally, accept that you're sometimes at the mercy of the Gaijin random number generator, where your 88mm cannon which could (in real life) reliably kill a Sherman tank at any angle, from over 1km away, simply bounces shots off a standard M4A2 while it mocks you with its' .30 cal.
(Apply in-game to squadron, we'll get you in)
