This research aims to compare the mileage and fuel consumption of a BAJAJ Pulsar 150 equipped with a 4 stroke SI Engine with and without the assistance of Hydroxy (HHO) gas through theoretical calculations and derive suitable conclusions. HHO gas consists or Hydrogen (H2) and Oxygen (O2) which are liberated during water electrolysis. The hydrogen gas produced by electrolysis of water is injected through a small opening in the engine intake manifold while the carburettor, fuel pipeline, and engine cylinder being unaltered. Topics from elementary Chemistry, Engineering Thermodynamics and IC Engines were used in the calculations. The specifications of SI Engine used in the Bajaj Pulsar 150 were also taken into account. The following results were obtained from the calculations when the engine was run with the aid of HHO gas: (a) 4.54% increase in mileage of the bike when operating the engine under maximum power condition. (b) 4.71% increase in mileage of the bike when operating the engine under maximum torque condition. (c) 74.2% decrease in fuel consumption, (d) No change in Air-Fuel(A/F) ratio