Re: How fast can you safely tube?
Like mentioned above, speed is only one part of the equation.
On completely flat water (no chop, ripples, or wake from other boats), and going in a straight line, I've found that you can match the speed up to the riders level of daring. Of course it's up to the driver/ spotters to ensure you don't allow your tubers to hit a rough patch at 45-50 or run over a partially submerged log.
If you're turning, note that the tube goes quite a bit faster than the boat; tighter the turn, faster the tube acceleration. The tube is also going to cross your own wake at this higher speed, so take that into consideration. I believe that this may be potentially the most dangerous tubing situation.
Rough water is another factor to consider. Adjust speed accordingly.
For skilled thrill seekers (my teenage nephews), I've found the best ride is start REAL slowly (generates a huge wake) then keep looping around tight enough to keep them in the turbulent wake. REALLY bounces them around. I bounced one kid maybe 8'-10' in the air, boat probably never went over 12-15 mph. They loved it, but note that these were 17 year olds that were doing flips off the top peak of the boathouse (close to 2 stories high).
Bottom line: match the ride to the conditions and skill/ age/ thrill seeking level of your tuber. The best ride is to take someone right up to the edge, but not past the point where they get truely scared. Too much & it will be a long time before someone that gets hurt or scared will get back on the tube...