Seems like the speedometer keeps failing and breaking at like 600-700rpm and I don't see why.

Seems like it could be at 80 radians per second maybe.

It is supposed to break at 2 times the set maximum measurable speed. You can change that speed by using an engineer's hammer on the speedometer.
Yes, the documentation in the manual is a bit lacking. I wrote the code for the mechanical multiblocks over the course of half a year or so, and wrote the documentation right at the end when I'd forgotten half of the little details.

In practice, I use the speedometer to measure how close to the flywheel's structural limit (The part of the machine where you placed for pure blocks of say Lead) in radian/s it is. Once it gets close I use a subtraction comparator to only allow a signal of 15 to get through, at which point I cut the power to it using things like redstone breakers from IE or simply apply a redstone signal straight to the generator in question. Since the multiblock loses speed overtime and you can drain power from it, eventually this redstone signal will get low again and then proceed to allow generators to run.