Towing explained: the numbers on the plate and what they permit

A car carries a plate on a door pillar, and the handbook repeats it. Between them they give four figures. Read all four: they constrain each other, and the one that runs out first is rarely the one people quote.

There is a fifth number, and it catches people out: gross train weight, sometimes printed as maximum combination weight. It is the most the loaded car and loaded trailer may weigh together. Subtract what your car weighs on the day — family, dog, full boot — and what is left is the real trailer allowance. It is frequently less than the braked limit, which quietly assumes you travel light.

A car with 378 hp — the most in this catalogue — is not automatically a better tow car than one with half that. Dragging a trailer up a hill is a power question, the easy one. Keeping the trailer behind you at motorway speed is a stability question, and that is the one that ends badly.

A trailer is a pendulum hinged behind the rear axle. Any disturbance sets it swinging: a lorry passing, a crosswind, a change of camber. Whether it dies away or grows depends on mass and geometry. A heavy car with a long wheelbase, towing a trailer loaded low and slightly nose-heavy over its axle, damps the oscillation. A light car with a heavy, tail-loaded trailer does not, and above some speed the damping goes negative and the snake builds by itself. Nothing in the steering fixes it then; ease off gently and let it settle.

• Load heavy items low and over the trailer axle, never at the tail. Weight behind the axle turns a small disturbance into a snake.
• Aim slightly nose-heavy. A common working figure is 5 to 7 per cent of the laden trailer weight, inside whatever maximum the car and the tow bar allow — the lower of the two applies.
• Use the laden tyre pressures from the handbook on both vehicles, not the everyday ones.
• Towing speed limits are lower than normal limits in most European countries and differ from one to the next. Look up the ones for the roads you will use.

1. 1

   Weigh the trailer as you will really use it

   Not the empty weight in the advert. The laden weight, with the awning, the water and the bikes. A weighbridge settles the argument.

2. 2

   Check it against the braked limit

   If the laden trailer is heavier than the braked figure, stop there. No amount of driving skill substitutes for the number.

3. 3

   Check it against the 85 per cent guideline

   Divide the laden trailer weight by the kerb weight. Below 0.85 is comfortable. Between 0.85 and 1.0 is permitted on many cars and demands respect. Above 1.0 is for people who tow for a living.

4. 4

   Do the gross train weight sum

   Add the car as you will load it to the laden trailer. If the total is over the gross train weight, something comes out, and it is easier to take it out of the car.

5. 5

   Subtract the noseweight from your payload

   The download on the ball is carried by the car, out of the same allowance as passengers and luggage. On a well loaded family car it is often what tips you over.

6. 6

   Check the tow bar separately

   The bar has its own plate, with its own maximum trailer weight and noseweight, either of which can be lower than the figure for the car. The lower one governs.

The six rarely agree, which is the point: the honest towing capacity of a car is the smallest number the exercise produces, not the largest on the plate.

Towing costs energy in two ways. Mass costs you on hills and under acceleration, and some of it comes back on the way down. Drag costs you continuously and gives nothing back, and a caravan is a large, square, badly behaved shape pulled through the air behind an object that spent months in a wind tunnel. At motorway speed it is most of the bill.

The planning figure most towing drivers use is blunt: assume a caravan roughly halves what you would get from a tank or a battery. A small flat trailer costs far less; a tall box costs about that much. Treat it as a planning assumption, not a specification, and find your own number on the first trip. What a car costs to run covers what it does to the monthly figure.

This matters most for electric cars, for two reasons. The first is arithmetic: the electric cars here quote between 225 and 655 km, and halving those leaves roughly 110 to 330 km between charges, which turns one long tow into a multi-stop day. The second is more awkward: most rapid chargers are laid out as parking bays rather than drive-through pumps, so you unhitch to charge and hitch up afterwards, and every stop costs more than the time on the screen. Charging explained covers why those stops behave as they do.

None of which makes an electric tow car a bad idea. A heavy car with instant torque and a low centre of gravity is a good thing in front of a trailer, and the finder will rank electric cars against a towing brief. It makes it a thing to plan around.

Mirrors

If the trailer is wider than the car, the standard mirrors will not show you the lane alongside it. Towing mirrors — the clamp-on extensions — are required in most of Europe when your view down either side is obstructed, and are the cheapest safety item in this guide. Fit them before the trip, and set them so you can see the trailer wheels as well as the road behind.

The licence

Licence categories for towing differ from country to country, and several European states have changed theirs recently. What you may tow depends on where your licence was issued and when you passed your test, so a rule quoted confidently by someone in another country may not apply. Check your entitlement against your own national rules before you buy a trailer. This guide states the physics; it cannot state your law.

Reversing

Reversing a trailer is not difficult, it is inverted, and the shorter the trailer the faster it happens. Two things make it learnable in an afternoon. Put your hand at the bottom of the wheel, so moving it left sends the trailer left. Then make small corrections early instead of large ones late. Practise in an empty car park before trying it on a campsite with an audience.

The shopping list is short: a braked limit above your laden trailer, a kerb weight comfortably above it, a gross train weight that leaves room for the family, and a tow bar rated for the noseweight you need. A long wheelbase helps stability and a hybrid or diesel drivetrain helps the refuelling stops, but neither outranks the weights.

Towing limits here span the whole range. Some cars are rated at 0 kg and may not legally tow at all. The median is 1,300 kg. The highest is 3,500 kg, the ceiling for this class of vehicle in most of Europe anyway. That spread is why assuming a large car will be fine is a bad habit: body style does not predict towing capacity, and the plate does.

Or set the use to towing in the finder and it will rank all 101 cars against it. The rest of the prepared answers are in the answers hub .