Compressed natural gas (CNG) is increasingly promoted as a lower-carbon alternative to petrol, yet important knowledge gaps remain regarding the influence of auxiliary loads such as air conditioning (AC) on retrofitted CNG vehicle performance and the limited availability of tailpipe idle emission data for carbon monoxide (CO), carbon dioxide (CO₂), and nitrogen oxides (NOx). This study addresses these gaps through a real-world comparative assessment of a petrol-powered passenger vehicle operated on petrol and on full CNG substitution, with minimal retrofitting to preserve baseline drivability. Road testing employed the Total Fuel Replacement method along mixed-traffic intercity routes to quantify fuel economy and operating costs, supplemented by flat-bed dynamometer measurements of wheel torque and power under AC-on and AC-off wide-open-throttle conditions. CNG operation resulted in an 18.84% reduction in wheel torque and a 33.31% reduction in wheel power under AC load, accompanied by a 15–20% decrease in fuel economy. Despite this, operating costs decreased by up to 48.13% due to the lower unit price of CNG. Idle emission measurements indicated substantial reductions in CO (80%) and CO₂ (35%), while NOx increased by a factor of five but remained low in absolute terms. Overall, the findings provide integrated technical and economic evidence that supports informed decision-making for broader CNG utilisation in passenger transport.