The magnitude of the solar wind speed decrease from the inner to the outer heliosphere provides important information about the interaction of the heliosphere with the local interstellar medium (LISM), leading to better estimates of the size and shape of the heliosphere and of the LISM density. In this study we combine observations from multiple spacecraft (ACE, Ulysses, and Voyager 2) near solar maximum with a multifluid one-dimensional numerical model to examine the solar wind slowdown. We find a decrease in speed of 53--62 km s-1 at Voyager 2 (~60 AU) with respect to ACE and Ulysses measurements or a 12--14% slowdown, the largest yet reported. The model results agree well with observations if we assume an interstellar neutral hydrogen density of 0.09 cm-3 at the termination shock.