It all depends on the gravel. I use Challenge Strada Biancas for my road / packed dirt / allroad tire. The 36mm Strada Bianca gives up very little speed on paved roads compared to 25mm Conti 4000S II's. And if I'm not racing, I figure it will let me work my way through any off-pavement surprises.
But for gravel races where I've never run the course? I'm not brave enough to take on an unknown gravel course with slick or slickish tires. This week and next -- on two very different courses I've never run -- I'm running Terrene Elwood 47's, just to be safe. For one of the two courses, I'm almost certain I would be faster on a 38mm tire that's slickish, but I won't risk it. I've heard the course includes some areas of bottomless talcum-powdery sand. For the other course, there will be multiple stream crossings, a route that runs in stream beds for a distance, and climbs/descents of over 20% grade. I wouldn't run a slickish tire there on a bet.
Last edited by FlashBazbo; 04-22-19 at 10:12 AM.