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In this study, experiments were carried out to evaluate fuel consumption and exhaust emissions of carbon monoxide (CO), oxides of nitrogen (NOx) and hydrocarbons (HC) with compressed natural gas (CNG) and gasoline fueled SI engine powered vehicles. Studies were carried out to evaluate optimum spark timing to operate both on CNG as well as gasoline mode for fuel consumption and emissions. In this connection, mass emission tests were carried out at various spark timings. Conversion efficiency of a 3-way catalytic converter was also evaluated at different air-fuel ratio for CNG operation. The results showed that exhaust emissions of CO and NOx were significantly less with CNG as compared to gasoline. Optimum spark timing for better fuel economy and lower emissions on gasoline and CNG modes were 10° before TDC and 24° to 26° before TDC respectively.