I want to learn how i can adjust combined gas-brake pedals. I want to use both gas and brake at same time. But, when i push brake pedal while holding gas, it makes car rpm get down.
(from memory so it may not be exactly correct) You need to go into the game controllers section of the control panel and untick the "use combined axis" box.
What exactly are you trying to do by pressing both gas and brake? If you are left foot braking to transffer wieght to the front wheels then just tap the brakes lightly or else you will certainly slowdown the car which explains why the RPM is going down.