Steam-Assisted Gravity Drainage (SAGD) is a remarkably successful process for the tar sands (oil sands). Two closely spaced parallel horizontal wells, injector above the producer, form a SAGD well pair. Steam is injected to provide heat to the reservoir oil and mobilize it. The low viscosity oil drains down to the producer under the gravity effect. Parallel well pairs 1000 m long are utilized in the process, spaced 100 m apart horizontally almost in all projects. In this work, an analytical model for the SAGD process is introduced by coupling heat and fluid flow and constitutive equations. A moving boundary, counter-current flow approach is used for the steam chamber rise and subsequent sideways expansion. The model is unique because it assumes the steam injection rate is constant and it permits modeling of the late phase of SAGD when adjacent well pair interference occurs. This leads to a reduction in heat loss to the overburden and a decline in oil production rate. This study examines the question of optimal well pair spacing in relation to the formation thickness and in-place oil. The effect of other variables on SAGD performance is investigated. A case study was performed using Christina Lake oil sand properties to show how the project performance varies under different senerios involving well pair spacing, reservoir thickness, steam injection rate, and steam quality. Results show that, in evaluating a SAGD pad performance, as the spacing is increased, the cumulative oil production decreases, with a simultanous increase in the cumulative steam-oil ratio at the same steam injection rate. However, a smaller portion of injected heat is lost to the overburden. It is concluded that a smaller well spacing requires more wells to deplete the whole pad area. On the other hand, a larger pattern well spacing affects oil recovery and heat consumption. Different conclusions are derived for the same pattern well spacing value using a single well pair model and pattern well pair configuration. Results also show that SAGD well pair spacing can be increased with an increase in formation thickness. The computational procedure is simple and makes it possible to examine a series of options for well spacing for a given set of conditions. This study presents for the first time an analytical relation between SAGD pattern well pair spacing and oil recovery.