I'm in the process of turning my '73 Motobecane Grand Record into a gravel bike. The paint has crackled on it so it's a fine platform for a rough stuff bike. It's 2 x 5 and I'm running a 50/34 nervar 631/633 crank and a 13-28 5 speed freewheel.
I'm running schwalbe marathon mondial 700 x 35c tires. The tires are plenty wide enough and there is sufficient clearance all the way around. They measure 35 mm and the tightest spot is along the seatstays as that is a bit narrower than under the front fork. I didn't do much research on the "right" gravel tire. These tires came to me as part of bunch of stuff I picked up in a package deal and they're fine tires. You aren't going to go wrong running schwalbe marathon mondials on rough roads. You can buy a pair of these for the same price as one Rene Herse Bon Jovi tire (less if you shop around) but they weigh right around 200 grams more per tire. Still the total weight of my Moto GR with these tires is 25 lbs which isn't bad.
