State Highway 66 begins at Business I-40 in El Reno (another old Route 66 segment), concurrent at this point with US-81. From this intersection, the highway heads due east. The highway briefly passes through a rural area after leaving El Reno, before transitioning into the Oklahoma City suburb of Yukon. In Yukon, the route runs along Main Street, and has a short concurrency with SH-4. SH-66 continues east, passing into Oklahoma City, where it becomes N.W. 39th Expressway. SH-66 has an interchange with the Kilpatrick Turnpike on the far west edge of the city. The highway passes just to the north of Lake Overholser. Near where the road crosses over the North Canadian River, the inlet to the lake, it crosses from Canadian County into Oklahoma County.