Actually I want to make a robot using 12 volt dc motors. So I want it to run it at a specific speed say 15km/hr when the wheel size is 2.5 inches.

Power is measured in Watts, Joules/second, Horsepower, and other units that can be interconverted.

Power - Wikipedia (http://en.wikipedia.org/wiki/Power_%28physics%29)

Joule - Wikipedia (http://en.wikipedia.org/wiki/Joule)

The power that each DC motor generates is found by multiplying the current it draws (amps) times the voltage (12 volts).

P = \, I \, V

Where P is the power in watts, I is the current in amps and V is the voltage in volts.

So, if you look at the motor and see how much current it draws, you can calculate the wattage of the motor. Alternately, you can measure the current draw.

Now comes the hard part. Newton's first law of motion says that a body in motion tends to stay in motion and a body at rest tends to stay at rest until a force acts on it. If you want to get to a specific speed, you can use this equation:

F = m\, a

Force = mass times acceleration. If mass is in kilograms, and acceleration is in meters per second -- per second (meters per second squared), the force required will be in Newtons. Newtons is a unit of force. A watt is a unit of power.

I was going to write a lot more, and I started doing so but I think you'll get confused. Basically, you need to figure out how much force is required to get the robot up to speed. You also need to figure out how much force is needed to overcome friction. In the absence of friction, Newton's law says that all you have to do is accelerate to speed and then coast. Unfortunately, we have friction in the real world -- lots of it.

Basically, your problem is soluble only if you know a lot more about what's going on. It's most easily measured after you build the robot.