For nearly a century experimental measurements of the velocities of detonations in gases have been found in general agreement with those of the Chapman-Jouguet (C-J) hypothesis predicting velocities, relative to the burned gases, equal to the speed of sound in the burned gases. This was further supported by the Zeldovich - von Neumann - Döring (ZND) theories predicting Chapman-Jouguet velocities for detonations in which the shock and reaction zones are separated. However, for a very fast reaction, the shock and reaction regions overlap and the assumptions required for the C-J and ZND theories are no longer valid. Previous work with the direct simulation method established conditions for forcing the reaction and shock regions to coalesce in a detonation wave by means of a very fast exothermic reaction. The resulting detonation velocities were characterized as ultrafast, as they were found to exceed the steady-state velocities predicted by the C-J and ZND theories. Continued investigation into the ultrafast regime has allowed for the further development of this inconsistency with theory by including a heavy non-reacting gas in the mixture. The resulting gaseous mixtures closely followed the C-J predicted behavior for slow reactions, and for very fast reactions were found to produce ultrafast detonations with a substantially greater deviation from C-J behavior.