OK , here is the math, it's not tough and will let you find good electric producing motors without too much problem
Lets say you are looking at a motor that 2600 RPM at 24 volts
Now divide the RPM by the voltage, 2600 / 24 = 108.33
you will need 108.33 RPM to make 1 volt
Now multiply the 'RPM per volt' times your planned Cut-In voltage
108.33 RPM times 12 volt Cut-In voltage equals 1300 RPM
There you go, your motor will make 12 volts when it reaches 1300 RPM
A proper Flooded Lead Acid battery is not really fully charged until it reaches 14.6 volts.
108.33 RPM times 14.6 volt Full-Charge voltage equals 1582 RPM
Too doggone fast.
That should be enough to figure out for yourself which one is a good generator.