Serious question, how far do you wear down your rear knobby tyre before you replace it? As much as it is nice to have fresh tyres, at what point do you say it's time for replacement? On a trip it might be a little different as you might stretch it out to get home instead of changing mid trip. Big trips I will normally start with a new pair and keep the old ones for run outs on day trips. Currently have a Motoz Rallz rear with about 5mm of knob left in the middle, I have a new one ready to go. Do I get another day trip out of it and risk getting spiked on a sharp rock, or say fuck it and fit the new on?