As long as there is traffic around then whatever their "average" speed is but basically just fast enough to not be in the way and slow enough not to run over anyone that pulls a bone headed move right in front of me. If there is no traffic and the ECM cuts the fuel at 100mph so I set the cruise on 95 and keep an ear out for radio traffic about any law in the area .
With my K10 on my flatbed, loaded w/ tools, spare tires & parts, water, etc, the weight is between 8200 and 8500lbs. W/ the Dodge 360 65's all she's got. 45-55 in 2nd gear on even the slighest incline, and 30mph in 1st gear on the steep hills. That's all she can do with the pedal mashed the whole time. When I finally do get a REAL tow rig, I'll probably pull 65-70.
Too fast most of the time. I usually do 75-80 on open highway depending on conditions and traffic with my Blazer (16-17K gross).
I tow at about 65-70 here in cali, and I could go alot faster, Ive got lots more power than needed, but I dont feel safe going any faster. Scott