To fill your NOS gauge when you have hydraulics, flip any car or SUV and hold A for about two seconds. Then, press any button to activate the hydraulics. Immediately, the word "Hydro" will appear on the right hand side of the screen
with a number underneath. The number should continue to rise and not stop. To stop doing this, release A and press Back to flip the car back over, starting with a NOS gauge filled to the overflow gauge.