The importance of tyre pressure: A message from Andrew
The two main types of tyre are P: metric, which is predominantly for passenger cars, and light truck.
P-metrics display the width, sidewall height as a percentage of the width, and the rim diameter in inches. For example: 225/65R15—is a radial ® 225mm wide, sidewall height 65% of 225 (146mm) to suit a 15in rim.
If changing wheel sizes, it is important to stay as close to the same overall diameter as possible , in order to maintain steering and suspension geometry and speedometer accuracy.
Larger wheels will need a lower profile (sidewall height).Light truck (LT) tyres are heavier duty. They are common on commercial vehicles and designed for carrying loads. They are usually the best choice for heavier caravans, as they will perform, handle and brake nearly as well as lighter-duty tyres, but will stand straighter and respond better to road irregularities under load.
LT’s feature deeper tread to evacuate water and mud, and if kept at the correct pressure, they will usually last longer (around 50,000km for a single axle, 70,000, for a dual). To calculate the load rating for a caravans’ tyres, you must know the maximum total weight of the van fully loaded (AGM).
Divide this by the number of wheels to determine the weight carried by each tyre.
Note: The load that can be carried by a tyre is determined by its pressure. Beware loading a tyre close to maximum, as rough conditions may require lower pressure ; therefore lowering the load capacity. Allow a comfortable margin.
(Extract from “Tyre choice”)