Rover Speedo Repair
Problem: odometer has stopped right at 100k miles. No more, No less. More specifically, the speedometer is fully functional still, but the odometer has stopped ticking over at 99,999 miles on the dot. However, the resettable "trip" odometer is working just fine. And also, this is in reference to the mechanical speedometers in pre-1996 Defender speedometers. This has nothing at all to do with the newer electronic speedometers.
Now, I cannot guarantee that this is the problem with YOUR speedometer, but it was the problem with mine. I also learned the hard way what to do, and what NOT to do in the process of fixing it. I ruined the speedometer portion of mine by touching parts I shouldn't have, in ways I shouldn't have. But I learned a lot about it in the process, so I will pass the knowledge to you as well.
Step 1: Removal
First thing is first, you need to remove the speedometer from the vehicle. Not a hard job, but not one I can take photos of. To do this, you need to remove the screws surrounding the dash panel, and gently lift it away from the dashboard.

There will be two light bulbs to remove from their white sockets in the back of the speedo, as well as the drive cable. The drive cable will simply pull away, but don't pull the cable. You need to pull from the connector itself. Otherwise you may ruin the drive cable. Also, be careful with the bulbs as well. The sockets are fairly fragile, and to fix them you need to take the whole speedometer apart. Yes, that is the point right now, but also remember not to break them when you put it all back in.

Oh, and another note. Be careful with the side panels that the dash removes from. These easily break, and things go downhill from there. One crack, and things get weaker and break more and more often until you are replacing a lot of dash parts.

Now, to actually get the speedometer out of the panel, there are two bolts extending out of the speedometer. Remove the thumbscrews from these two bolts, try not to lose the lockwashers. On each of these bolts will be a metal bracket, also remove and save these two brackets. The speedometer should now lift out.
Step 2: Removing the lens
This is where things get tricky, and once again, be careful! First thing to remove is a rubber gasketting ring between the bezel and the main body of the speedo. This should just lift right out, but after almost 15 years it may be pretty dried out and easily damaged. This isn't a critical part, but does help to keep the inside of the lens from fogging up.

Then the hard part. You need to pry the rolled lip up around the back of the bezel in order to remove it from the body. This will take a good stiff pair of pliers that can fit under there, takes a good amount of work to get it lifted up. And you need to get about 2/3 of it undone to get the whole thing off. Goof this up, and you risk damaging the lens or making visible scratches that will show after it all gets re-installed. But once you get enough of it loosened up, it should all just lift out. There is also another gasket on the back-side of the lens, putting it back is a real pain in the butt, so just set the whole lens aside and don't fiddle with it.


Step 3: Removing the speedo mechanism.
Disclaimer: From this point on is where you can really goof things up! I'll point out a few "don't" things here. These are the important ones that will permanantly ruin the speedometer, and will necessitate sending it out for repair and re-calibration, which ain't cheap.

I'll start with the needle. Don't play with the needle. Don't touch the needle. Don't spin the needle around. It's tempting, I know, but don't. It's attached via a very delicate spring, exactly like those used as balance springs in mechanical watches and clocks.

What you DO need to do is look at the back of the metal cup that the gauge is mounted in. There will be two hex-headed screws. Remove these screws, and the gauge mechanism will come out. While it drops out, protect that above-mentioned needle from bonking into anything. Or you will be sorry.
Step 4: Ticking over the Odometer.
Ok, disassembly is done now, don't freakin undo anything else here. Especially do NOT twist those three little metal tabs on the back of the mechanism, as this can possibly mis-align the magnetic coupling, and again this will kill the whole gauge.

What you do need to do, is get a pencil with a nice healty eraser on it. You need to carefully take the eraser of that pencil, and where the arrow is in the photo, push that part of the odometer drum upwards a few times. This is where the problem is, as it has a very weak spring inside the anti-reverse mechanism. Most likely it isn't broken, but it just isn't strong enough to spin that extra digit to click it over to 100,000 miles. Just to make sure, click it up a few times to go a few miles to say 100,003 miles or so. Technically you are now fixed, hopefully, and we are back to re-assembly.
Step 5: Re-Assembly

First things are first, if you need to fix the lighting, do it now. Should be pretty simple.

Next, make sure that the reset lever for the trip-odometer is back in place.

Then place the mechanism back into the housing. Same two hex-headed screws to secure it back in there. Be careful not to scrape the side of the housing, the paint flakes off pretty easy in there.
Now, place the lens back on the front. Make sure that the pin for the trip-odometer reset is properly aligned through the gauge face and properly in the hole of the reset mechanism.

Now, this is the tough part. You need to re-roll the crimp of the metal bezel. Don't just clamp it with some pliers, that will scratch the hell out of it. Don't forget that gasket you took out, easier to put it back in at this point.

Put the gauge face-down on some wood, with some fabric between the wood and gauge. Drill a hole in the wood for the reset button. Now, take a short and strong wood dowel, and with a hammer use that dowel to tap the back of the bezel down into place again. Be gentle, take your time. You need to work the whole thing back down fully, or the gauge won't fit properly fit into the dashboard.

To reinstall in the dashboard, reinstall in reverse of the removal procedure. Brackets with lockwashers and thumbscrews. Press-fit the drive cable, push until it snaps into place. Two lightbulbs. Screw the dash-panel back into place. Try not to break the side panels.