vintage motor
Join Date: Sep 2008
Location: Tepic, Nayarit, Mexico
Posts: 1,595
Bikes: 48 Automoto, 49 Stallard, 50 Rotrax, 62 Jack Taylor, 67 Atala, 68 Lejeune, 72-74-75 Motobecanes, 73 RIH, 71 Zieleman, 74 Raleigh, 78 Windsor, 83 Messina (Villata), 84 Brazzo (Losa), 85 Davidson, 90 Diamondback, 92 Kestrel
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If the gravel roads you'll be riding on are not too rough, then 27x1-3/8" tires (about the equivalent of 700x35) are probably fine. I would try that first, and if you don't like it then look for other options. I don't think switching to 700C is going to gain you enough clearance to use wider tires. I have an International from about the same time period, and the chainstay clearance limits me to 700x35. Maybe the Gran Sport's had more clearance but I wouldn't count on it.
If you switch to smaller diameter wheels, that's going to be a fairly big expense, and you'll only be able to use slightly wider tires, 38mm or perhaps 40mm. Not many bikes from the early 70's will take 650Bx42 tires.