Unfortunately, since wagons and wagon trains were often unpredictable in their movement -- usually slower than the general would've wanted -- it is somewhat hard to give a firm rate or a firm rule, as, by definition, it would be too predictable. So, the following suggestions should be treated as a guideline rather than a hard rule. First of all, I would start with the assumption that - by design and doctrine - wagons were intended to be able to keep up with a marching column of infantry.

Hence, I would recommend by starting off by using the infantry column march rate - 18 inches on road, 14 in open, etc. Then, for each turn roll one die, and that is the number of inches of movement that is lost due to ornery mules, temporary tie-ups, wobbly wheels, or whatever. Overall, that is not much of a problem unless the wagons are trying to go through woods or worse. But, that assumes: good weather, full teams of six mules, and well-maintained wagons - so that is as good as it gets.

However, for each additional hassle, such as rain or mud, less than full teams, or poorly maintained wagons, add an additional "negative" die, and very quickly the wagon train, even on a road, becomes more of a problem, than a resource. A nice additional touch - though it does increase the bookkeeping -- is that should the negative dice total be greater than the allowed movement for the terrain in question, keep track of how much the wagons went "into the hole" and insist that that deficit be made up on following rolls before the wagons can move. Using this rule, it is easy to see how a wagon train, should it decide to decide to risk going over broken terrain in bad weather, could wind up stuck for the remainder of the scenario. The intent of these guidelines is to recreate the frustration of wagon train movement, and despite the fact that without them the army is borderline nonfunctional, not a few generals would give the order to, "Burn the Wagons!"