Edward

Personally, I will perform preventative maintenance when the consequences of a failure *costs* me more than would have otherwise cost me (that's cost in dollars or inconvenience, like being stranded). Now I presume that when you say "replaced the DME" you mean replace the relay which, if it were to fail, results in a dead 993. That IS smart PM. Side note: this DID happen to me, BTW. But I did the less-than-smart thing. I kept the original relay in, and put the updated replacement in the trunk. So at least I was not SOL, but was inconvenienced by having to pull over with a dead car, then had to go through the mental machinations/worry, then remembered the DME relay ...shazzam! You're obviously smarter than I am. And yeah, I have another spare in the trunk.

So IMHO, with the odo gears I don't see a point in fixing what isn't broken, and simply addressing it when the need arises. Not to mention why dig into the speedo and potentially break something when all was still working fine.

Edward

Fred's 964

The original gears will fail because of the material they are made of and the lubricant used by Porsche. I replaced the ones on my 964 and it was obvious that the old gears were soft after 16 years. The new ones are made of a different material and you don't lubricate them. A better quality gear for sure.
Mine broke within 10 miles of my home, so I didn't drive it until I replaced the gears. I was afraid when I disassembled it the numbers would turn, so I noted the mileage to put it back with the correct mileage. While replacing the gears, you can see you would have to really intentionally tamper with the unit, as stated before, to change the miles. For the 8 or so miles it was broken, I was not about to tamper with it. I will divulge this if I sell it. Remember, there is always the chance someone tampered with your odometer before you got the car, a consistent mileage history with the condition of the car is a better indicator of whether it is a car I would own. Most owners are honest and would not know how to change the mileage unless someone tells them how to do it. If someone told me how to do it, I don't think I would share it on an open forum. Not being critical, that's just me. I suppose with the internet, anything is available if you look for it. For me, changing mileage is seldom a repair, most who change it are probably not making the numbers higher...

clib

I would wait till it fails for all the reasons Edward stated. and it may never fail. You definitely mark up the retaining ring disassembling it and most likely get a scratch here or there. Not a huge deal and particularly in the context of fixing something - but preventative for something that doesn't seem to affect most cars doesn't seem worthwhile.