A friend was given a Cannondale SR400 in like-new condition. I tuned it up for him and he told me he doesn't want to ride it on the road but use it on the crushed gravel bike trails near his home. Obviously, the 25mm road tires on the bike aren't going to cut it for that use.
According to the Sheldon Brown chart, I can go up to a 32mm tire on the rim but I think that's barely adequate. I have a wider-rimmed rear wheel I can give him if he's willing to buy a front wheel to match it so he can go up to a 35- or 38mm hybrid tire. Will I run into any fit problems with this approach?
In the alternative, if I just put a wider tire on his current rim, I'm concerned with finding a matching Presta tube. Is it an absolute must to match tube size to tire size or can I use something marked for a smaller tire?