2 of my mates are lorry drivers 1 has a scania, the other has 4 lorries, 2 volvos 1 scania and 1 merc, on a windy day against the wind the mpg for my mate with the scania drops from 8.5/9 to around the 6 mpg mark and my other mate the scania drops from 7.5/8 to 5.5 mpg the volvos drop from 7.5 to 5 mpg and the merc drops from 7.5 to 4 mpg, so the wind can have quite an affect.