Yup, your Tire Rack searches are part of the hassle I encountered.
One listing at Pirelli for Pzero had no 16" tires listed. See below.
I also found news of Pirelli revising the Pzero tires in 2016, and I'm thinking they dropped some sizes at that time. Which might explain the problems with searching and results.
And may also bring into concern the manufacture date of older sizes still in stock but not currently offered.
Plus, the various lines of Pzero confuses things, too.
Larry
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