Back in my early 60s, I led a group ride series of mixed fixed, SS, and geared bikes. I used 42 X 17 SS with 23mm tires. The climbs were tough - up to 10%, but i could coast on the descents and keep up on the flat with the geared bikes as long as I was sitting in. I remember long stretches of 135 cadence in the paceline, so at least 25 mph, maybe more, I don't remember. I couldn't hold over 20 mph pulling. I was one of the weaker riders. I'm a good descender though, so what I lost on the climbs I could make up on the descents and get back on. With a SS it worked out well, really improved my spin. I used my ordinary road bike and just didn't shift. The series didn't make me any faster on rides when I could shift though. I was surprised at that. I think that's called the rule of specificity.