In this paper we propose a variable speed control strategy based on a new Variable-Length cell transmission Model (VLM). The VLM differs from the standard Cell Transmission model in that only a limited number of (variable length) cells are used. Road network is subdivided into several sections which are assumed to be composed of a downstream congested cell followed by a free upstream cell. Both cells have variable lengths and are described by two lumped densities (one congested, the other free). One more state describing the length variation completes the model for each section. The paper also introduces an associated optimal speed control design based on the proposed VLM. The method is illustrated on a closed ring road and is shown to optimize the traveling time per turn.