When we lift our Jeep Wranglers and put larger tires on we suddenly change the ratios that tell us accurately how fast we are going. Bigger tires will cause the Jeep Wrangler to go faster than what the speedometer indicates we are going. This is because one complete revolution of the tire previously would go a particular distance. Now with a larger tire we are going a further distance with the same full revolution of the tire. Our actual speed is now faster than what the speedometer tells us.

When we get larger tires we need to get a new speedo gear to tell the Jeep's computer the correct distance each revolution of the tire takes us.

Another thing we do to our Jeep Wranglers is change the differential gearing. Mine is currently 3.73. Soon I will change this to 4.88 to accommodate my 33 inch tires. When we change the differential gearing we also need to change the speedo gear to make up for the gearing ratio change we made in comparison to the old ratio.