Astraist wrote:That would be a challenge for me, given that the tyres on my GT3 come with around 4.5mm when new!
That's different. Unlike perfomance tyres, road tyres come with 8 to 8.5 millimeters of tread depth. So we see eye to eye. Four years are a good maximum age limit (including storage time), but some tyres age within three and a-half, three or even two and a-half years. They will probably last a bit longer in the UK than under the mediterranean, where most tyres dry out after three years.
I also agree that awareness to the state of the tyres is important, so I have the habit of checking the tyres visually (by a brief walk around the car) each time I enter the car. The alignment is something I check each year, or after 6,000 miles, when I get the tyres removed, inspected all around and rotated.
Just to be clear, the GT3 tyres are road tyres, just performance oriented (in this case Michelin Pilot Sport Cup). They are actually quite capable in the wet, provided one keeps the speed down. They even coped with this downpour at the Nurburgring!
Very few tyres in the UK come with more than 8mm of tread - most are actually between 7 and 8mm when new.