To clarify things further, here's an example
Say the company tire size is 155/70/R13 - wherein the Contact patch is 155mm, the Aspect Ratio is 70 and the rim size is 13". According to the Plus-Sizing concept, the ideal upsize would be a 165/65/R13. This would put the minimum stress on the key components while giving a wider contact point for the road.
Guru's please correct me if i'm wrong about this.
Cheers