A well known failure mode of the electronic type odometer is a broken gear inside the odometer drive. This drive basically is an electric motor spinning a epicyclic gear that in turn drives the odometer. The speedometer usually remains unaffected.

That's the basics done.
A short while ago, I noticed it was my turn to be the "victim" of a broken gear. So I took the odometer apart, and yes, a tooth had broken off of the little 25 teeth gear.

Then I went looking for replacement gears and found out there are two versions: one with 25 teeth (like I have) and one with 26. But no one would specify when what gear was used and in exactly what instruments.

Obviously, I was wondering why this is, but I couldn't find that out. Only that it was the case with SOME odometers. It didn't seem to matter if the odometer was in miles or kilometres, or what its K-number was.

So I went digging some more. I found here that the 26 teeth gear would make the odomoter run differently. The larger gear would make the odometer go slower by " almost 3%". But strangely, an example was given in mph (speed) and the one thing that gear doesn't affect is the speedometer in the electronic version. With the odometer gear broken, the speedometer will happily keep working as it always did. So that just seemed wrong.

I also couldn't find any basis for that 3% figure. If there were any difference, it would have been almost 4%.

Then I realized that the gearing of the odometer drive is an epicylic gear.
I went on to read about epicyclic gearing ratios, only to find out that the broken gear was the one gear that did not affect those ratios.

Rather counter intuitively, it DOES NOT matter if that little gear has 25 or 26 teeth. Only the number of teeth on the driving gear (on the motor) and the number of teeth on the clear plastic housing (61 teeth to be exact) determine the ratio (together with what part of the epicyclic gear is held stationary).

So for all wondering why there are two versions: I still DON'T know. I do know it doesn't matter which one you replace the old one with. Often, the 25 teeth version seems to be the cheaper option.

Epicyclic gearing

The tooth count in the stationary or ring gear, the molded clear styrene housing, determines ratio that makes a K9800 different from a K10042 or a K6241 for example. Same with the 92/93 K39200/K40168/K24964 versions.

I don't know the tooth count for yours (K6241 or K24964) but you may be able to compare it to the K9800/K39200 (61 teeth) and the K10042/K40168 (63 teeth) by looking for the equivalent to the Z61 or Z63 marking molded into the clear plastic.

Like you, I couldn't find any official word on which 25 or 26 tooth gear got used with which odometer, but the size of the stationary gear would be larger corresponding to the tooth count, so a larger gear might mesh better. I wondered if the 26 tooth gear was a replacement solution to wear, because (1) I've never seen one in a unit, and (2) loose mesh might have been one hypothesis to explain the short life of the originals.
--
Art Benstein near Baltimore

It seems like a lot of suppliers are only interested in plugging the hole with a part, rather than making sure the part is of high quality. -Benski

K24364 combined with Z61 ring gear. This 245 would originally have been on 185R14 tyres.

I would expect the finer pitch of the 63 teeth ring gear to mesh better with the 26 teeth planet gear, (and the Z61 with 25). But if these combinations were always the same, we probably wouldn't be having this confusion now.
The possibility of finding a 26t planet gear also only seems to be confined to the 1992 and 1993 model years (of the 240, I don't know about the 700/900-series).

Skandix says this in the remarks section of the parts:

- 25 teeth (Skandix 1021067) or 26 teeth (Skandix 1062109) at the Speedo.
Both versions may occur: The 25 teeth cogwheel is the most common in Europa, the 26 teeth cogwheel is the most common in North America.