Don't most dual sport tires wear out faster than street tires?
Not necessarily. Metzeler Tourances lasted me a pretty long time. IIRC, I think I got ~15K mi out of a set. Semi-knobbies like the TKC 80 or Karoo T do have a fairly short life though, but not that short when compared to some of the softer sport tires.
Are dual sport tires really that much better than street tires on mild off-road riding (i.e. well graded gravel and dirt roads)?
No, not at all, again unless you're talking about a semi-knobby. Street-oriented DS tires are marginally better, but when compared to the likes of the TKC 80 they are really all about the same. For maintained dirt roads, a pure street tire would do just fine.
What other tires have similar qualities of the Avon Azaro/Storm that are good on the Wee?
I don't have experience with those tires, but if I were to stick largely to pavement I would not hesitate to fit Tourances again. They're a pretty good all-round, all-weather tire.
- Jet Jaguar
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