So many opinions and none of them are really the "wrong" one. The correct speed to travel while towing is the one you are comfortable with. Several posts on the subject are bordering on bullying. Not cool. If you don't like the fact that I and others choose to tow at 63 mph, or the reasons we do so, then just pass me. Otherwise tough beans. When I'm not towing I keep up with traffic just fine and have the utmost respect for those who choose to tow at a reasonable speed.
Bottom line is that I've towed pretty much all over the place except the east coast, and yes I have towed on CA Interstates. Never felt like it was a problem to tow at 62/63 mph. A lot of other traffic on the Interstates travels much slower than I do.
BTW, aside from the Bay area, Milwaukee/Chicago area drivers are far worse than any in CA.