Okies .. to try and help, and make it simple ... using MY DETAILS as an example ... but YOUR car and YOUR van should provide the same info.
From the car VIN plate on the door pillar.
TOP figure 2200 Kgs
SECOND figure 4075 Kgs
1) Top figure is Manufacturers Max Permisable Weight (also known as Gross Vehicle Weight GVW)
2) Second figure Max allowable train weight
3) Subtract 1 from 2 ... 4075-2200 = 1875 Kgs - this is the maximum allowable trailer weight, or the vehicle is deemed overloaded.
4) From the caravan plate (by the door) - MPTLW 1650 so as this is lower than (3) the combination is legal on the road.
However, as 1 + 4 = 2200 + 1650 = 3850 the driver MUST have a B + E (or equivalent) in order to drive.
The above is the info the POLICE will go on regarding license and legality
VOSA and their scales - either portable or at a weighbridge will look at different criteria.
From the car plate ..
5) Third figure 1080 - Max front axle weight
6) fourth figure 1150 - Max rear axle weight - NOTE from the handbook this can increase to 1200 when towing - so I'll use 1200 for the calculations.
The astute among you will note that 5+6 = 2280 which is greater than the max 2200 allowed ... IF the rear axle is loaded to 1200 the front axle must be decreased to 1000, as the 2200 CANNOT be exceeded .... actually a tricky thing to do !!
So on the scales ... the weights of 4, 5, 6 will be taken as measured .. if the published limits are exceeded you are breaking the law. They will look at each figure in turn, (axle loads) then at 4+5 (car load) then at 4+5+6 (train weight)
It is not unheard of for the car to be well within limits and the 'van to be high, but not by stupid amounts. VOSA will usually insist that loads are re-distributed to make the unit legal (and safe) rather than prosecute
So thats the law ... many folks have heard of the "85%" rule .. which is NOT a rule !! it is the Caravan Club recommendation, no more, no less and is NOT based on plated weight of the car at all, it based on the "kerb weight" which according to my handbook is 1700KG for my car.
It is designed to make the "worst case" scenario of light car + heavy van as safe as possible for the inexperienced tower, so is based on kerbweight (1700) and max van weight (1650) .. as you can see, using those figures I am at 97% !!! Generally deemed a "bad thing" by the CC .. however, in mitigation .. I have a gurt big LPG tank usually full in the boot, my van is not at its max, and I've been towing for 25+ years so don't class myself as "inexperienced".
You can also see that there is no way an omega towing another omega on an A-frame or trailer can ever be legal, unless something drastic has been removed from the towed car (engine /gearbox etc etc)
Sorry its so longwinded, but that is MY understanding of the law and its ramifications, hopefully it is not too far wrong and may help.
Tow safely
