That being said on average this time of year depending on the Water Level i.e. the total amount of water in the system based on flow rates should be between 5-8 miles an hour.
Last time I put a GPS on it as well as the Tanana from the Wood to Nenana we were at 6.5 MPH on average. However that was not from the Dalton to Rampart it was from Eagle to Circle concerning the Yukon.
I was at 6 - 7 from Tanana to Galena according to my GPS so that all sounds right. If you do some research on here seaulater did it from the bridge down to the Koyukuk and lists his times on the hunting forum.
Lots of variables, time of year, volume of water...leaving the Dalton, you are dropping off the Yukon Flats and the gradient is steeper to Rampart. Won't be the same speed as going through the flats....