Tractor unit adverts often lead with horsepower. The axle layout decides whether the unit will do the work. A 4×2, a 6×2 and a 6×4 can share a cab and a badge and still be the wrong lorry for your trailers, your weights and your routes.

Approved tractor units for sale on Trucksalot show the configuration when the seller has entered it. If the advert only says “tractor unit”, ask before you assume a mid-lift 6×2. This is the usual UK fleet layout, and it is not the only one.

What the numbers mean

The first number is the wheel positions. The second is how many of those are driven. A 4×2 has one driven axle. A 6×2 has three axles and one driven axle, most often with a lifting mid axle ahead of the drive axle on a UK fleet unit, though tag axles exist and should be named in the advert. A 6×4 has two driven axles. They are not a list of trim levels. They are different vehicles.

Confirm which axle lifts, whether it is a mid-lift or a tag, and whether the unit is on the air management your workshop already knows. “Automatic” is not a gearbox specification. Ask for the make of the box and whether a retarder is fitted.

Match it to the trailers you have

Fifth-wheel height and the trailers you already run belong in the same conversation as the axles. A unit bought because the cab looks right can still sit at the wrong height for a curtainsider, a tipper trailer or a tanker. Ask for the fifth-wheel brand and height, and whether it is fixed or sliding.

Weight and route decide the rest. General UK haulage is why so many fleets buy 6×2 mid-lift units. A 4×2 can suit lighter work. A 6×4 is there when you need the extra driven axle. If you are comparing a used unit with a new order, take the same list to the quotation: axles, cab, coupling, tanks and tyres. The note on specifications that matter on a new truck is the new-vehicle version of that list.

What to have in front of you at a viewing

Take the advert, the MOT history and a note of your heaviest regular trailer. Check that the mileage on the dash sits sensibly against the test history, as described in checking the MOT history. Look at the drive-axle tyres, the fifth wheel and any air leaks before you talk about the cab trim.

The price on the advert is excluding VAT unless the seller has said otherwise. How to read that figure is covered in price excluding VAT. Once the axle layout is right, the rest of the negotiation is about a lorry you can actually use.