Front Axle Gear Ratio on 06 Trailblazer Ext Ls GM RPO Axle Codes To observe the beam gear ratio of whatsoever GM vehicle, locate the RPO sticker (typically located in the glove box) and write down any codes starting with F, M, or H. So, find those codes in the table below. NEW! - Need assistance with GM RPO axle codes? Check out the new comments section below! GM RPO Beam Code Notes These RPO codes are used to idicate a specific type of differential and/or ring gear size. Not all applications will have one of these codes but when they employ, they will be listed in improver to an beam code found in the main RPO axle code listing below. RPO