The odometer is actually very easily recalibrated so long as your Jeep is not a Rubicon. It takes 5-10 minutes from start to finish to remove the old speedometer gear and snap the new one into place. It literally unsnaps and snaps back in. This also recalibrates the odometer so it reads correctly. You only need a 1/2" wrench to remove the bolt that holds the speedometer gear housing to the transfer case.
Here's how and the chart shows what speedometer tooth count to use to get your speedometer reading correctly... Changing Your Jeep Speedometer Gear
One last comment... pay close attention to where it describes how to rotate the speedometer gear housing to match the number of teeth on the new gear. BUT that is not a precise thing. You will likely need to rotate it a bit more in one direction or the other to get the new speedometer gear to mesh with the pinion gear inside the transfer case. So if the speedometer doesn't work after installing the new speedometer gear, don't worry about it, you only need to rotate the housing a bit so the two gears mesh properly with each other.