The paper presents the results of a multi-year analysis of the environmental and economic impacts of the operation of vehicles powered by compressed natural gas (CNG) compared to Diesel vehicles. The study is based on real data collected by the EPIC (Environmental Performance Integrated Centre) project, which monitored fuel consumption, CO₂ and NOₓ emissions, as well as fleet operating costs during the period 12/2015 - 6/2020. The aim of the paper is to analyse the environmental and economic impacts of operating CNG vehicles compared to Diesel vehicles, based on the data collected during the period under study. The results of the regression analyses confirm that CNG vehicles achieve significantly lower GHG emissions and at the same time bring significant financial savings. Specifically, CNG vehicles emitted approximately 122,504 kg less CO₂ than their Diesel counterparts, a reduction of around 28 %. Emissions of NOₓ were reduced by 65 %, and particulate matter (PM) emissions were approximately 90 % lower for CNG vehicles. Additionally, the regression analysis of fuel costs shows that Diesel fuel costs increase at a significantly higher rate than CNG, making CNG a more cost-effective solution, especially for high-mileage urban transport. Forecast models for the 2015–2030 period indicate a 22 % projected reduction in emissions from CNG vehicles compared to just 7 % for Diesel, and operating costs for CNG are expected to decline further, while Diesel costs continue to rise. CNG thus proves to be an effective transitional fuel in the context of a sustainable mobility and energy transition strategy in the transport sector.