No-load RPM is generally linear with voltage, so you should expect 21-22k RPM at 12V. It usually comes along with 2x the no-load current, which means 4x the power and heat (2x current * 2x voltage).
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (no, it's NOT all-wheel-drive--look HERE
for more details!)
Pink Rocket Princess Mustang - 15mph, 36V, 4x 775 motors, homebrew PWM controller rev 2....
Hurricane - 24V w/50A step-down controller, 775 motors