In countries with limited financial resources and access to new technologies, as well as difficult environmental conditions, the process of renewing urban transport infrastructure is a complex process. An example is Africa, especially Ethiopia. Ethiopia is assessing compressed natural gas as a transport fuel for intercity buses currently powered by ageing diesel engines. This study reports a controlled before and after comparison of a single 12 m coach tested in its original Euro III diesel configuration and after replacement with a 6.7 L spark-ignited CNG engine equipped with stoichiometric control, exhaust gas recirculation, and a three-way catalyst. Chassis dynamometer tests were performed using the world harmonized transient cycle (WHTC) and a locally defined intercity cycle, followed by repeated on road measurements on the Addis Ababa-Adama corridor using a portable emissions measurement system. Distance-based emission factors for carbon monoxide (CO), total hydrocarbons (THC), nitrogen oxides (NOx), particulate matter (PM), and carbon dioxide (CO2) were determined from diluted exhaust measurements. Under both cycles, the CNG configuration yielded lower NOx, particulate, and CO2 emissions than diesel. Hydrocarbon emissions increased under both cycles, while CO increased under the WHTC and on the road but decreased slightly under the intercity cycle. On-road measurements confirmed the same directional trends. Fuel consumption and operating cost were evaluated from measured fuel use and local retail prices. A fleet scenario was derived from the measured emission factors and stated activity assumptions. The results indicate that engine-replacement CNG conversions reduced NOx and particulate emissions in Ethiopian intercity service.