* Check the vehicle's documentation: The original window sticker or owner's manual might list the axle ratio.
* Look at the axle tag: The differential housing usually has a tag with the ratio stamped on it.
* Perform a calculation: Jack up one rear wheel, mark a spot on the driveshaft and the tire, rotate the tire one complete revolution and count the number of driveshaft rotations. The ratio is calculated from this information.
Without more information about the specific El Camino, it's impossible to say definitively what its rear end ratio is.