I just bought a new Ironhorse enclosed trailer for my bikes yesterday. While I was at the factory I asked the owner about tires. Her statement was to replace them every three years - period! She said it didn't matter if they went 1000 or 10,000 miles per year that they just seem to crack and wear and get unreliable after that time period. I am even more grateful that my 3500 mile drive last year on eight year old tires went so well. My other instructions were to keep the tires aired to 65# cold and to check them every morning before I hit the road. At roughly $100 something a tire, this seems like cheap insurance.