Depends on what kind of gravel you are riding, how much you weigh and how fast you want to go plus other factors. Michigan fire trails as an example have quite a bit of sand and bigger tires are great in that environment, 45mm would a good choice if you don't want to get off and walk as much. B-roads with ruts and big gravel I want 40mm or larger. Regular dual lane A gravel roads that have fine gravel sub 40mm works well.