Some answers to your questions:
1. That makes things easier!
2. The wheel size seems reasonable. What are the tires made from? Something with good grip will do a better job than something that spins due to a lack of friction between the tire and the surface.
3. That's heavy for a robot! What constitutes the majority of the weight? Is it the chassis, the batteries, or something else? Making your robot lighter is the easiest way to reduce the amount of power required to move it.
4. 1m/s is quite fast for a robot (it's a realistic walking pace), or even a basic R/C toy, especially one that weighs 11kg. Even 1/3 that speed is quite fast for a line follower. This just makes me more curious about how much power you're putting through your motors. What can you tell me about your batteries and how you're switching power to the motors? How many motors are you using?
5. The low torque you've described seems inadequate to move the mass of 11kg of your robot.
also i've heard that high gear ratio means high torque and hence u can control it better when u follow the line , is this correct ?
Yep, you've got that right. Bear in mind that, for a given power in watts, there is a trade off between torque and speed. A higher torque motor will move a heavy robot, but if you use the same power as you're using at the moment with your present motors, the robot will move slower. If you want more torque and the greater speed you've described, you're going to need bigger motors. If I had a better sense of what you're using already, I'd be better able to suggest something else. If there's no way to avoid the weight and speed requirements you've mentioned, something along the lines of power window motors are in order.