The Chevrolet K-10 pickup is a four-wheel drive model, indicated by the "K" in the nameplate. The more common "C" model is rear-wheel drive. "K" models usually come with low gear ratios, such as a ring gear and pinion with a ratio of 4:10. This means there are more gear teeth to keep constant power applied from the engine to aid in pulling. A high-ratio gear set, such as 2:73, has less gears and wider gaps between the gear teeth. High ratio gears are used for low-power, high-speed applications.
Raise the hood of the truck and use a flashlight to locate the Regular Production Option (RPO) code plate. On older models it is riveted to the firewall under the hood. On other models it is a plastic, laminated decal stuck to the inside of the glove-box lid. This list of codes details all of the equipment the vehicle came with at its time of production.
Read the list and look for "G" codes. This code group refers to gear ratios. GM uses over 100 codes for the "G" group, however, examples to look for are "GS4," or "GS5." These are low gear ratios used in truck applications and represent 3:70, and 4:11 gear combinations.
Verify your findings by jacking up the truck and installing jack stands under the frame. Remove the rear axle cover and the final drive cover with a 3/8-inch socket wrench. Catch the axle lube in a drain pan.
Paint marks on the gears, both the drive gear and the driven gear, called the pinion and ring gears, with a paint pen and use these marks as a starting points. Count the number of the teeth on both gears and divide the larger number by the smaller. The result is the gear ratio and should agree with the code. If not, the axle gears have been altered and are not original to the vehicle.
