I don't have any facts to verify but what I've gathered over the years is up to the point when econodyne was "introduced" Mack engines were 2100 rpm high torque rise engines that used 5 and 6 speed transmissions. Rated HP was at 2100 rpm. The econodyne engines turn slower and have their rated HP at 1800 or 1750 RPMs to save fuel and less engine noise / driver fatigue. My guess is the there are slight differences in the fuel system and engine internals (cam / pistons) between a 2100 RPM Maxidyne and an 1800 RPM Econodyne. Again no facts. Just anecdotal info...
67RModel's post in Mack meaning was marked as the answer