The motivation of manufacturers to pursue higher productivity and low costs in the fabrication of optical fibers requires large diameter silica-based preforms drawn into fiber at very high speed. An optimal design of the draw furnace is particularly desirable to meet the need of high-volume production in the optical fiber industry. This paper investigates optical fiber drawing at high draw speeds in a cylindrincal graphite furnace. A conjugate problem involving the glass and the purge gases is considered. The transport in the two regions is coupled through the boundary conditions at the free glass surface. The zonal method is used to model the radiative heat transfer in the glass. The neck-down profile of the preform at steady state is determined by a force balance, using an iterative numerical scheme. Thermally induced defects are also considered. To emphasize the effects of draw furnace geometry, the diameters of the preform and the fiber are kept fixed at 5 cm and 125 μm, respectively. The length and the diameter of the furnace are changed. For the purposes of comparison, a wide domain of draw speeds, ranging from 5 m/s to 20 m/s, is considered, and the form of the temperature distribution at the furnace surface is kept unchanged. The dependence of the preform/fiber characteristics, such as neckdown profile, velocity distribution and lag, temperature distribution and lag, heat transfer coefficent, defect concentration, and draw tension, on the furnace geometry is determined. Based on these numerical results, an optimal design of the draw furnace can be developed.