146 Driving and operatingWhen driving downhill, drive in thesame gear as if driving uphill anddrive at a similar speed.Adjust tyre pressure to the valuespecified for full load 3 205.Trailer towingTrailer loadsThe permissible trailer loads arevehicle-dependent and engine-dependent maximum values whichmust not be exceeded. The actualtrailer load is the difference betweenthe actual gross weight of the trailerand the actual coupling socket loadwith the trailer coupled.When the trailer load is beingchecked, therefore, only the trailerwheels - and not the jockey wheel -must be standing on the weighingapparatus.The permissible trailer loads arespecified in the vehicle documents. Ingeneral, they are valid for gradientsup to max. 12 %.The permitted trailer load applies upto the specified incline and up to analtitude of 1000 metres above sealevel. Since engine power decreasesas altitude increases due to the airbecoming thinner, therefore reducingclimbing ability, the permissible grosstrain weight also decreases by 10 %for every 1000 metres of additionalaltitude. The gross train weight doesnot have to be reduced when drivingon roads with slight inclines (less than8 %, e.g. motorways).The permissible gross train weightmust not be exceeded. This weight isspecified on the identification plate3 195.Vertical coupling loadThe vertical coupling load is the loadexerted by the trailer on the couplingball. It can be varied by changing theweight distribution when loading thetrailer.The maximum permissible verticalcoupling load (80 kg) is specified onthe towing equipment identificationplate and in the vehicle documents.Always aim for the maximum load,especially in the case of heavytrailers. The vertical coupling loadshould never fall below 25 kg.When measuring the vertical couplingload, ensure the drawbar of theloaded trailer is at the same height asit will be when the trailer is coupledwith the towing vehicle loaded. This isparticularly important for trailers withtandem axle.Rear axle loadWhen the trailer is coupled and thetowing vehicle is fully loaded(including all occupants), thepermissible rear axle load (seeidentification plate or vehicledocuments) must not be exceeded.National regulations regardingmaximum speeds for vehicles towinga trailer must be observed.