Several recent studies have indicated that TCP performance degrades significantly in mobile ad hoc networks. This paper examines how bad TCP may perform in such networks and provides a quantitative characterization of this performance gap. Previous approach typically makes comparisons by ignoring the inherent dynamics such as mobility, channel error, and shared-channel contention. Our work provides a realistic, achievable TCP throughput upper bound, and may serve as a benchmark for the future TCP modifications in ad hoc networks. Our simulation findings indicate that node mobility, especially mobility-induced network disconnection and reconnection events, has the most significant impact on TCP performance. TCP NewReno merely achieves about 10% of a reference TCP?s throughput in such cases. As mobility increases, the relative through-put drop ranges from almost 0% in static case to 1000% in highly mobile scenario (mobility speed is 20m/sec). In contrast, congestion and mild channel error (say, 1%) have less visible effect on TCP (with less than 10% performance drop compared with the reference TCP).