My introduction to riding gravel was the 120 miles on the Denali Hiway on my Jamis Renegade, 700x40 tires with too much air in them. From pavement to reasonably packed and civilized gravel, there was a gradual transition to less civilized aggregate, until there was a section of fist sized pebbles shaped like little pyramids with very pointy tops. Then, gradually it transitioned back to a normal gravel surface. I'm thinking about returning to the legendary Hiway for an encore, and I debate about going with a plus bike. But maybe 700x2.1 is the sweet spot for a mix of gravel and pavement.