  We establish the optimal convergence rate of the hypersonic similarity for
two-dimensional steady potential flows with {\it large data} past over a
straight wedge in the $BV\cap L^1$ framework, provided that the total variation
of the large data multiplied by $\gamma-1+\frac{a_{\infty}^2}{M_\infty^2}$ is
uniformly bounded with respect to the adiabatic exponent $\gamma>1$, the Mach
number $M_\infty$ of the incoming steady flow, and the hypersonic similarity
parameter $a_\infty$. Our main approach in this paper is first to establish the
Standard Riemann Semigroup of the initial-boundary value problem for the
isothermal hypersonic small disturbance equations with large data and then to
compare the Riemann solutions between two systems with boundary locally case by
case. Based on them, we derive the global $L^1$--estimate between the two
solutions by employing the Standard Riemann Semigroup and the local
$L^1$--estimates. We further construct an example to show that the convergence
rate is optimal.
