DISCLAIMER: THIS GUIDE IS FOR TRACK CARS ONLY. Rolling back the odometer on a street car is ILLEGAL. Not to mention unscrupulous if you do it with deceitful intent. I suggest you look at it this way, if your car is registered with your state and driven on public roads, you cannot do this. On the other hand, if you own a dedicated track car that is trailered when on public roads and not required to register then by all means proceed as you wish.
With that said, this is something I find immensely useful on a swapped chassis. Setting the odometer to zero when installing a new ca18det or sr20det makes it super easy to keep track of the overall miles you have put on the car.
There are just a couple of notes I want to mention. First of all, pay careful attention to the way the odometer fits together before you disassemble it. This will make it very easy to reassemble if you somehow get things out of order. Also, the small white tumbler gears need to be aligned with the numbered gears when reinstalling. Once reinstalled, spin the blue gear on the side and verify that there is no binding. Furthermore, rolling the odometer back one to 999999 and then forward to 000000 will snap the gears in place and will help with the alignment.
Finally, click on any picks for a larger view.