Rubber degrades over time and heat cycles, so even if a tire still looks great, it can't compare to new rubber. If you like riding aggressively in the twisties, like I do, you should NEVER leave an old tire in place and just replace one... but that's just me. Always in pairs, regardless of wear. My life is worth more than a $100
, but to each his own. What I do is reduce front tire pressure a little for better grip (if needed), and that also helps equalize wear more evenly, killing 2 birds with one stone. Haven't had a chance to ride my new Connie, which I'll do at 42 psi at both ends initially, and see how it grips. The lowest I'd go is 40/38, but I always ride solo, and only weigh 155# (well, 150# now due to chemo). And most likely will probably do 42/40. With a fully-loaded bike (2-up, very heavy rider, etc), I'd obviously never deviate from 42/42 (or even higher). You need to experiment a little to end up with the best pressure for your riding style and total weight.