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The Z1 Star Spec is the quickest of the lot, when it comes to street tire classes (treadware 140 minimum). They aren't as widely available as the RT-615 (sizes).
The RT-615 has been around for a few years (2005 introduction) and has been the tire all others have been measured against, for grip and longevity.
The new Kumho XS seems to be in the same grip level, but lifespan and grip towards the end of life are still uncertain at this point, since it's such a new model.
The one thing that the RT-615 has going for it, while the intial grip isn't quite as high as some of the newer models, it doesn't seem to fall off as bad as some of the others.
They do take a bit longer to get fully up to temp than a few others, but in the end, the heat cycling doesn't affect them quite as bad.
When new, there are tires that are faster the the RT615, but towards the end, those same tires struggle to keep up with like-worn RT615's.
All street tires will get greasy when hot, that's unavoidable without killing overall lifespan.
The RT615 is also usually at the lower end of the pricing scale, making them a good choice for all-around use, particularly for those on a budget.
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