As a general guide the advice is to change the caravan tyres every 5 years and 7 years maximum but that all depends on the state of the tyres. If the caravan is standing in the same position most of the year and exposed to hot sunshine during the summer months then the tyres will create flat spots thus leading to cracks and reducing the life of the tyre. The cracks are only visible on the outside so the inner sides needs to be checked as well so the wheels have to be removed in order to be able to conduct a proper inspection. If the air pressure is not maintained then this will also shorten the life of the tyre creating cracks usually around the wheel rim.
I have 2 sets of wheels. The tread rarely gets worn enough to need replacing.
Every 5 years i swap the tyres and relegate the old ones to winter storage and as emergency spares. Although they could be upto 10 years old. If i did have to use them it would be at a fairly low speed and replaced ASAP.
Don't forget to check your valves for cracks as well. I checked my tyre pressures a few years ago and couldn't believe how corroded and cracked the valve rubbers were
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