Originally Posted by
msu2001la
The CyclingNews article I linked above discusses this. It says "Zipp is doing tests on each tyre and making a recommendation."
They interviewed Continental in that article as well, and they indicate that they agree with ETRTO on this topic (which seems to contradict the chart on the product page for the GP 5000).
ETRTO is just guidance. As far as I know there's no requirement for anyone to adhere to those guidelines. My point with mentioning all of this is just to highlight that 28mm tires on a 25mm hookless rim is pushing the limits (or beyond them if you listen to ETRTO) and this combo should be approached with caution.
Personally I'd be comfortable running a 28mm tire on a 25mm Zipp wheel in part because they state that they've tested this combo and it's safe. The other part of this is that I run a 28mm tire at 60psi, well below the 73psi limit. If I were pushing up against that 73psi limit and/or using a wheel brand that isn't offering similar assurances, I'd definitely be going with 30mm.
GCN Performance on Youtube covered this and while I am no for hookless, it's important to remember these are just recommendations. However the recommended tire is now 29mm which just leaves ENVE tires? Or lik you mentioned, you can just jump up to 30mm tires. I have reiterated this countless times and that's hookless is fine but for gravel or MTB but just because "x" amount of people bought 303s doesn't mean your "average" cyclist gets hookless and you always run the risk of them over inflating and using the wrong tire