Summary form only given. TCP congestion control can perform badly in highspeed wide area networks because of its slow response with large congestion window. The challenge for any alternative protocol is to better utilize networks with high bandwidth-delay products in a simple and robust manner without interacting badly with existing traffic. Scalable TCP is a simple sender side alteration to the TCP congestion window update algorithm. In this paper we numerically evaluate the congestion control method of scalable TCPs and its impact on other existing TCP versions. The general purpose of this work is to study the behavior of scalable TCP not only in situations where it was reportedly performing good but also under more extreme conditions. We also revealed whether scalable TCP maintains an acceptable fairness towards highspeed and/or regular TCP versions. Fairness of scalable TCPs with different round trip times are also investigated.