The demand for alternative technologies to power internal combustion engines is increasing every day, as companies seek sustainable solutions that aim not only at a cleaner environment, but also at tangible economic advantages such as reduced fuel consumption. In Brazil, a large fraction of transport modes, especially freight transport, uses diesel as fuel, which led government agencies to implement strict regulations for pollutant emissions, such as Euro VI in Europe and the Proconve P8 in Brazil. In this context, compressed natural gas (CNG) offers a mature, clean, sustainable alternative contributing to the reduction of pollutant emission. Thus, the main objective of this work is to evaluate performance parameters such as specific fuel consumption, efficiency and economic analysis for extra heavy trucks fueled by CNG as an alternative to diesel in real applications in the country's freight transport. For the study we used two commercial engines, 4 stroke, turbocharged, 6 cylinders with a volume of 12.9 liters, one using diesel as fuel and the other using CNG, it is important to note that the engines are similar, with the same manufacturer. What differs is the form of operation, one works with the Diesel cycle, with compression ignition, and the other uses the Otto cycle, with spark ignition. The engines were tested on a dynamometer bench where their thermal efficiency and performance were compared. Based on these results it was possible to conclude that the CNG-fueled engine can be a replacement for diesel engines with a reduction in operating cost, but with a reduction in performance.