I have a 4 wheel trailer that sits in the driveway, I haven't used it for 5 years and the two tyres exposed to the sun are very badly perished, the side adjacent to the fence has far fewer cracks. In the garage I have a car that had new tyres fitted in 2013, it's only done 150 miles since and the tyres are like new. Admittedly my trailer tyres were made by an unknown Far East company but the ones on the car in the garage were a lot more expensive and made by Coker so maybe quality and UV play a bigger part than a lack of movement?