It depends on what unit you have. I run a Depth Raider and from everything I've seen, the DR and Subtroll are usually pretty close, but the Fish Hawk reads around .5 miles faster at the same actual speed. My usual target down speed on the DR is 1.8-2.2/2.3 most days, whereas I hear 2.5-2.7 most often for guys running the Fish Hawk. I will take some experimenting to find the sweet spot for your unit. I would suggest starting on the low end and gradually increase your speed until you start getting bites and keep slowly increasing until you stop then back it back down until you are getting bit again. Eventually you'll determine the target range for your unit under most conditions.
The only time this wouldn't apply is if you are offshore targeting steelhead, they like a much faster troll than kings do. I honestly don't think you can troll too fast for steelies, as long as you are running lures that run right at those speeds. That's what makes NK's in particular such good steelhead spoons, They kinda hang dead a slower speeds, but run great at even over 3mph. A lot of the R&R Superlites also have a huge speed tolerance.
Tim