How do you determine the rear axle ratio in your Dodge RAM 2500 Cummins Turbo Diesel?

Answer 1

Jack up the rear of the truck so that both rear wheels are about an inch off the ground. Place jack stands (two) under the rear axle tube near each tire. Block the front tires with four pieces of small 2x4 boards. Place the transmission in neutral.

First we will check to see if the rear end is open or has a limited-slip differential: Grab the right rear tire with both hands and rotate it clockwise hard. If the left rear tire turns in the opposite direction you have an open differential (boo hoo). If when you try to rotate the right rear tire it doesn't want to rotate or catches or grabs, you have a limited-slip differential (yeah). Now we will determine the gear ratio: Crawl under the truck with a grease pencil or piece of chalk. Place a mark on the inside of the right rear tire near the pavement and also place a mark on the pavement. Then place a mark on the drive shaft near the rear universal joint. Now grab the drive shaft with both hands and start rotating it. Take note of how many times the drive shaft turns to make the tire mark come back to the same spot. If you turn the drive shaft slightly more than four turns you have a 4.10. If you turn the drive shaft slightly more than than three and a half, you have a 3.55.