It really does depend on so many things. On a "fast" geared 4x2 in low I had one that I wound out to 15 mph (at 3200 rpm) every time I shifted her. My truck now (a slow 4x2) wound out to 3000 rpm is 5 mph. Most Kodiaks aren't geared that low - that's for the old babies. 15 mph is a little tall, but that setup only put me at 50 in fourth low (New Process four speed). If he really had it wound up good (I think redline is just under 4000 on them) I would say 12 mph is a fairly safe bet.
It would be easier just to do the math like someone said and then do a sample box.