Let's see here, a quick dig finds an article saying that a horse and wagon would be good typically in a city for about 20 miles a day
http://books.google.com/books?id=Vv--Pf ... #PPA268,M1
Now, that is in the city, where congestion might be an issue, but on the other hand, the roads would typically be better.
So, 1 Mile is about 17 hexes (a bit less actually), so ideally, a day's work should be around 340 hex. Now, assuming it works 14 hours a day, that's around 8 hexes per turn. That's in city conditions. Currently, a wagon can go 12 hexes on a pike, or 6 on a secondary road. So, the question is, would a secondary road qualify as better conditions than the city?
To me, that fast analysis shows me that the rate of advance of a supply wagon is in the ballpark (or even a hair high on pikes).
*shrugs*
Major General Gary McClellan
1st Division, XXIII Corps
AoO,USA