What is the factory gear ratio for a 1981 Chevy El Camino on stock carrier with ring and pinion 7.5?

There's no single factory gear ratio for a 1981 Chevy El Camino with a 7.5" rear axle. The ratio varied depending on the engine, transmission, and options selected. Common ratios included 2.73, 3.08, 3.42, and 3.73. You'd need to check the vehicle's original documentation (owner's manual, build sheet, etc.) or physically inspect the axle tag (usually located on the differential housing) to determine the exact gear ratio.