Well its pretty simple, my axle has picked my trailer speed for me! My truck runs at 2,800 RPM when going 65 mph empty, so I would tow around 2,300-2,400rpm putting me in the 60mph arena.
However the trailer should let you know where it is happy and is stable. You will hit that high speed where it bounces too much on bumps or pot holes, or sways, or tugs ******* hitch left to right, or does not slow down to well. So it will tell you what speed is reasonable and safe.
Ditto on what SJwelds said. Start out safe and slow for a couple dozen miles, speed up a little bit for a long section, do it again, and maybe push it a little for a short stint. You will know where its comfortable to tow.
My boat is big and heavy so hard to tow fast. Also a little harder to slow down. The extended trailer bounces the back end up and down over large bumps in the road. So 60mph is the goto speed.
My big tex car trailer tows a little wonky, does not pull super straight, wonders, and is hard to get to a good tongue weight with my jeep on it. So I do not like to pull faster than 65mph.
Now my Triton 20 foot aluminum deck over snowmobile trailer pulls like a dream. Pulls straight, does not sway, brakes good, hitch is tight, I do not get too worried about the speed even when loaded. However I choose to never go faster than 70 when permitted.(Out of state with a newer truck that can.) If something went bad, I want to know the truck towing could hold it on the road. Not whip the truck around like a rag doll.
Lastly many folks forget what the lever on the brake controller does. It applies only the trailer brakes. If you get into a sway at higher speeds, use the lever gently to apply the brakes. It will tension the two vehicles and get you straitened out. I see a lot of trailers jacknifed on the road, I think many did not know, forgot, or simply failed under pressure to try to do that. Then panicked hit the vehicle brakes to slow down, causing more sway, and then a jacknife or accident.
Safe pulling!