John, ditch the Dunlop.
Where is the spare mounted? On the back or underneath?
A little longer than 9 years ago, admittedly, there were issues with people bringing the spare into use and just buying 3 new tyres. We established (unofficially) that the age of the tyre plus the heat it was subjected to when mounted in the sun on the back of the tailgate, was resulting in some pretty full on failures. These were all Japanese Dunlops which I had experience with but I imagine other brands like Bridgestone and Yokohama would have seen the same issues. Probably wouldn't admit it though.
Put simply, the original set would wear out with no problems but the spare, when put on the road, had a limited life expectancy.
I had another instance about 10 years ago where a guy had a tandem trailer on which the two tyres on one side had failed spectacularly in service. All four were a good brand (forget which) and were fitted at the same time some years previously.
Talking to him we worked out his trailer was parked most of the time beside the house where the two tyres which failed were in full sun all day.
