You think it's half, we've been playing with 1/3 less life and 1/4 less cost. I find the ao50 is a more progressive tyre under foot.
Life through the tyre seems similar, if anything the ao50 still works until all the tread is gone, while the Z221 falls of a cliff before it looses it's last tread patches.
The GTR has C50 Z221 on it for this year, interested to see the lap pace comparison.