The mileage covered within Switzerland is read off the tachograph that is fitted in almost all vehicles which are subject to the fee. A fee is not charged separately for trailers but in conjunction with the tractor unit. In this case, the rate is calculated according to tariffs for maximum permitted weight on Swiss roads. This maximum permitted weight may be lower than the sum of the maximum total weight of the tractor unit and the trailer since it depends on the type of tractor unit and is subject to the general national weight limit. The national weight limit is 40 tonnes.