I second the comments above of “it depends”. Not sure if you’re referring to Linday rigs, spinners, and/or crankbaits.
The range Mark posted covers all of those. Live bait rigs in that .8-1.something depending on presentation and the fish’s mood.
For Cranks 2mph-3mph is a wide range that covers most scenarios. Speed trolling is a different animal that I don’t have experience with.
One odd thing we learned. The motor we were using wouldn’t get below 2.6 or so. We kept thinking that gee wouldn’t it be nice if we could get down to 2.2mph? I’ll bet we’ll catch more fish.
Turns out there were problems with the motor. Once they were solved it was down in the 2.1mph-2.2mph. So off we go trolling at the slower speed. A couple times we kicked the throttle up to 2.5mph and caught fish. Not being stubborn we listened to the fish and found 2.6-2.8 was the magic that day.
It’s too bad we can’t ask the fish each day what speed they want the baits