My example was in a specific gear (50/11). At 60RPMs that combination (at least on 700c, 25mm tires) gives 21.4mph, 90rpm is 32.4 or something like that. You could be in a different gear, at a different cadence - as long as the wheel is turning at a rate that would get you whatever speed on the road you'll get the same resistance from the trainer.

The fluid in fluid trainers is non-Newtonian (think corn starch and water). The more force you put on it the denser it gets. Once temperature settles, the speed of the impeller creates a predictable resistance. CycleOps has a graph with several of their trainers on their site which is reasonably accurate for speed vs. resistance. Mine runs a little higher, but I use a power meter.