Knowing the axle ratio for your 1999 Chevrolet Suburban is required if you want to change ratios to either improve towing and pulling power, by moving to a lower (numerically higher) ratio, or improve highway mileage, by moving to a higher (numerically lower) ratio. It is also necessary if you need to replace the ring and pinion gears. The 1999 Suburban uses either the General Motors 10 or 12 bolt rear axle which has a removable differential cover. Removing the cover will allow you to calculate the gear ratio.
Park the Suburban on level ground. Shift the transmission into neutral. Place wheel chocks in front of each front tire. Raise the rear of the Suburban with a jack and support the right and left axle tubes with jack stands. Place a drain pan beneath the center of the axle, under the differential.
Remove the bolts that hold the differential cover on the back of the axle. Gently insert a putty knife between the cover and axle housing in several places to separate the cover from the housing. Allow the gear oil to drain into the drain pan. Remove the old cover gasket and scrape any remaining gasket material from the cover and housing. Wipe the cover and housing with a shop rag.
Place a dot of liquid eraser next to one of the teeth on the ring gear. This is the large round gear that sits vertically in the axle housing. Place a dot of liquid eraser on the pinion gear. This is the small gear at the front of the axle housing that the drive shaft connects to which engages the ring gear.
Turn the drive shaft to rotate the ring and pinion gears. Count the teeth on each one. There will be between 10 and 14 teeth on the pinion gear and 28 to 40 teeth on the ring gear. Write down the number of teeth. Use a calculator to divide the number of teeth on the ring gear by the teeth on the pinion gear. For instance, if you have 39 teeth on the ring gear and 10 on the pinion, divide 39 by 10 to get 3.90. Some calculations will give you a number with more than three digits in it, such as 3.07685. Round this figure up to 3.08.
