I’ve been on 38c GravelKing slicks since ~August, run tubeless. Width on a 17mm ID rim is 39.5 on my cheapo digital verniers.
They feel really good to ride, are pretty light @ ~350g each and are reassuring in the way they grip, even in the wet. From this point of view I really like them.
However, the puncture resistance for me has been terrible. More than a puncture a month means that during ~1/10 rides I get a flat. Most of my rides are commutes from one side of Manchester to the other by going south of the airport on country lanes. I puncture as frequently in that setting as when I go directly across the city. All punctures seem to be of the size that should self heal but don’t. I keep the Stans fluid used topped up, always shake the bottle well etc. to ensure that the little plugging particles make their way in, but to no avail. It’s really frustrating, as I can’t rely on the tyres.
I’m going to give some Maxxis Re-Fuse 40c’s or Marathon Supreme 40c a try before going back to the ultra-reliable Marathon Plus I took off for this experiment. They’re heavy and draggy (extra training?), don’t feel as good or grip as well but it is becoming apparent to me that, for my location/routes/style at least, you either fit Plusses or better be prepared for flats. Gator Hardshells were good for me in the past at ~3 punctures/year but they’re not available in ‘big’.