[KiwiEV]

Mate, I had the exact same question as you a few weeks back and I was scratching my head trying to use the calculation table that came from EVAmerica.
In the end I found this link was able to answer my max RPM questions.
Also, according to EVAmerica, the max RPM of the FB1-4001A motor is 5600RPM from memory. As long as you know your gearbox's gear ratios then that link will sort you out nicely.

 

[KiwiEV]

Yep, I'm currently figuring out a way to attach a (very) little generator to the motor's tailshaft to generate current to move the tachometer.

I disconnected the tachometer's tiny needle "motor" input from the built in circuit board in the tach gauge. I ran the input straight past the tach circuit:

Then I re-routed it straight to a low voltage input; this is the result of a 1.5 volt AAA battery:

As you can see, 1.5 volts gives the tachometer just over 4000 RPM. I have a little motor which I connected to my cordless drill and put a meter across. I estimated the drill was spinning at about 2000 RPM and it produced 0.75 volts. I'm expecting that once the little motor is connected to the tail of the drive motor, it will generate up to 2 volts, maybe more.
I have a potentiometer which I will connect into the circuit so I can fine-tune the tachometer's results.

Now this is certainly not going to be accurate throughout the rev range, but as long as I can get it nearly accurate closer to the motor's redline then I'm happy.