Over the past few decades, tetrafluoroethane (TFE, R134a Freon)-based gases have been widely used in the operation of phenolic resistive plate chambers (RPCs) in many high-energy experiments. However, TFE has a high global warming potential (GWP); therefore, a search for new eco-friendly gases to replace traditional TFE-based ones is now unavoidable. In this research, we present cosmic-ray test results of a prototype RPC for the SHiP (search for hidden particles) experiment using 1.6- and 1.4-mm-thick RPC electrodes containing mixtures of various gases, including 1,3,3,3-tetrafluoropropene (HFO1234ze), CO 2 , i C 4 H 10 and SF 6 . We compare the performance data gathered with these new gas mixtures with those gathered with a traditional TFE-based gas used for RPCs in compact muon solenoid (CMS) and a toroidal LHC apparatus (ATLAS) experiments. The addition of CO 2 to the HFO1234ze-based gas was found to be fairly effective in reducing the working-point high voltage (HV WP ) for RPC operation. The results of our experiments lead us to the conclusion that adding 40% CO 2 or less, when combined with HFO1234ze-based gas, is conducive to reliable detector performance for SHiP single-gap phenolic RPCs.