The transition to low and zero-carbon fuels is crucial for reducing greenhouse gas (GHG) emissions from internal combustion engines (ICEs). However, fuels such as renewable natural gas (NG) exhibit low chemical reactivity and laminar flame speed, posing challenges for achieving rapid and efficient combustion. These challenges are further exacerbated under lean or diluted conditions, which are necessary to mitigate NOx emissions but often result in reduced combustion efficiency and prolonged burn durations. Enhancing in-cylinder flow speed is a promising technique for accelerating combustion and thereby improving thermal efficiency. This study quantifies the impact of flow speed (Uflow) and the resulting turbulence on flame initiation and propagation in lean methane combustion at air-fuel ratios of λ = 1.2, 1.4, and 1.6 using a rapid compression machine (RCM). A custom designed flow chamber is used to generate flow speeds ranging from 0.7 m/s to 96 m/s across the spark gap under the same mixture density and temperatures. CONVERGE CFD simulations are employed to quantify the induced flow speed and turbulence levels. The combustion process is characterized by determining the burn durations corresponding to 0–5% (flame initiation) and 5–90% (propagation) of the mass fraction burned. Additionally, shadowgraph imaging is utilized to qualitatively observe the flame propagation process. The results indicate that moderate increases in flow speed shorten burn durations and elevate peak pressures. The optimal flow speeds to minimize burn durations are identified as 33 m/s for λ = 1.2 and 23 m/s for λ = 1.4. At λ = 1.6, no optimal flow speed is determined, as none of the tested conditions indicated that increasing flow speed further extended the burn duration within the ignitability limit.