Car and trailer tyres must have at least 1.6mm of tread over the central 75% of their width for their entire circumference, although a tyre with under 3mm offers little grip in the wet. Tyres must be free from deep cuts and bulges (which indicate they are breaking up) and it is illegal to use cross-ply and radial tyres on the same axle. (Make sure that the tyres you fit to your trailer are of the correct rating.)
Additional Tyre information:
See links for detailed tables on Tyre Load Index, Speed Symbols and Pressure Ratings.
Tyre sidewall markings:
The illustration below provides an explanation of the markings currently found on a tyre sidewall.
* do not apply in the U.K.
Retreads - must also show either 'RETREAD' or 'REMOULD' on the sidewall, together with 'BSAU 144' in place of ECE Type Approval Mark.
Aspect ratio - ratio between the height of the sidewall and the width of the tread, expressed as a percentage. In the example above, the figure is 70%.
Nominal section width - on radial tyres this is expressed in millimetres, on cross ply tyres, inches are used.