Sorell, Wargaming Developer, has posted a new updated table with tank distribution since Update 9.18 was introduced. Because the information was in Russian, I’ve created the same table but in English so everyone from other regions can understand it.
The biggest change is the fact that Tier III tanks no longer fight Tier V, meaning Tier I, II and III have a -1/+1 matchmaker, and Tier IV is -1/+2, except for Heavy tanks that have -1/+1. Starting on Tier V the match maker is full -2/+2 for every class.
Source: World of Tanks Russian Forum