Road class has nothing to do with speed limits. You are referencing a table I wrote in the wiki and if you see below I say that you have to tag e and f roads as primary if no b road and according to the official data. You can open the data if you scroll down to the section some maps and click on primary road network. As for highways and trunks again I have a note that if a road is classified as A road but speeds are 80 then it should be trunk