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In the drive against air pollution in Delhi, diesel buses were replaced with CNG buses with gas cylinders mounted on the chassis below the bus floor. A series of gas leaks and consequent fire in some buses raised concerns about their fire safety. In this work, leakage flow from a moving bus (2-D, transient) was simulated using a CFD package. As natural gas is lighter than air, buoyancy effects were included. A vertically downward leak in the low pressure system at low speeds causes the gas to move towards the bus front while pushing upwards against the bus floor; a leak in the high pressure system at high vehicle speeds, however, ejects the gas towards the road from where it is convected into the wake. The mixing of natural gas with air was also obtained and regions within flammability limits were identified.