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1. My car is smaller than your's and it is not working good as yours. Which variables, constants i have to change if i scaled your car ?


I recommend you start by placing your car somewhere above the ground and let it drop. Try different gravity settings until it looks realistic.
Next, set wheel and body mass to a low value and drive the car around. If the car bounces a lot, increase the mass values. If the car has a tendency to flip over then reduce the chassis mass and increase wheel mass. To adjust the friction settings see if the car slips sideways when in a curve or while the handbrake is set. More friction reduces slipping but can lead to undesirable results if too high- just set it to a low value and work it up. Chassis friction is not too important- if the car is a "low rider" you should set that friction to a very low value, otherwise it can be high.
On a level field the car should not be rolling much on its own. Add wheel damping if it does. If the car becomes unstable at high speeds you will have to reduce the maximum speed by changing the max angular value. Should the car be slow in getting moving or the wheels spin in place then you will have to adjust the wheel torque that is applied.
If your car gets stuck in walls you need to reduce the PH_CHECK_DISTANCE parameter. In my demos I use 0.1 for this. The simulation will generally run better at a higher physics framerate (default:70, 140 is better) but requires more CPU power for that.
For best handling the car should be wide, not too tall, with large tires and a chassis that has some ground clearance; in other words a golf cart will be hard to simulate.

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2. In multiplayer mode, i have created 2 physics car. And it is working good on server, but on client it does not accept key controls. Please help me to solve this example ? Here is my code

I am sorry, but I can not debug projects, unless they specifically demonstrate a potential bug. If you remove everything that is nonessential to the problem at hand, I will check it.