Fuzzy logic systems are widely used in literature for heuristic knowledge based system operation. However most of these systems operate on multiple input to provide a single output. In this paper we have introduced a scheme for implementation of single input single output (SISO) fuzzy logic approximation of PI controller to control a two input single output (TISO) system. In most systems controlled by PI or PID controllers, there exists to input variable namely error (e) and change in error (Δe). However, there exists an opportunity to view these controllers as SISO system as Δ e = f(e), hence it is eventually resource effective to approximate these controllers using single input fuzzy variable. This paper introduces this concept and implements it on a model of Automatic Cruise Control (ACC) in an inclined plane with effect of multiple disturbance forces.