Determining the transfer function of black box?

I have a model of a car with two inputs, namely Throttle and Brakes, and one output, which is car velocity.I dont know the mathematical model of this structure. My intention is to find out the transfer function of this model. At present a rough idea is to apply some throttle, say 4% of full value, and get the output values of speed. Now knowing the input graph and output graph, how do I estimate or calculate the transfer function in Simulink.