After installing bigger tires, it's important to change the speedo gear so your odometer and speedometer read correctly. The rear of your transfer case has a speedometer gear housing. You must figure out what speedo gear you need, and thankfully there are charts online that will aid you to figure out the correct tooth count. Easy job to do, and the part itself is cheap. Go to a dealership and tell them what your gear ratio in your diff is and what tire size you are running now . They can provide you with the correct part.