1. A shift control system for a vehicular automatic transmission having different transmission positions, comprising: vehicle condition detecting means comprising a vehicle speed sensor for sensing vehicle speed of a vehicle, and an engine load sensor for sensing engine load of an engine of the vehicle, and fuzzy control means comprising setting means for setting, for each transmission position, a membership function of said vehicle speed and a membership function of said engine load for defining a fuzzy set, and determining means for determining a transmission ratio from said vehicle speed, said engine load and said membership functions by fuzzy inference, and producing a shift control signal for controlling shifting of the transmission, wherein said condition detecting means further comprises acceleration sensing means for sensing an acceleration of the vehicle, rate of engine load change sensing means for sensing a rate of change of said engine load, and running resistance sensing means for sensing a running resistance of the vehicle, and wherein said setting means of said fuzzy control means further sets a membership function of said acceleration, a membership function of said rate of engine load change and a membership function of said running resistance, and said determining means further uses said acceleration, said rate of engine load change, said running resistance, and said membership functions of said acceleration, said rate of engine load change and said running resistance for determining said transmission ratio.