One of my A units runs quite a bit faster than the other one. I installed a DCC decoder in both engines, and tried to tweak them with DecoderPro to get them to be more similar in their behavior, however, I was never able to get them to work well together. When they ran as a consist at a fairly high speed they were somewhat in the same ballpark, so I used them as such on our club layout one time. However, at slower speeds the second unit was noticeably pushing the leading A unit. It, therefore, became quite hot. One of the club members mentioned that I may have a low gear and high gear unit. I decided to order 4 of the "low" gears (just in case I wanted to replace them on all four trucks of the two units). This package is what I received from American Models.
To replace the gear you pretty much have to disassemble the engine. In this photo below, I have removed the bottom cover plate of the gear tower, removed the wheels, disconnected the electrical wires from the tower, and removed the gear tower itself by removing the lock nut on top. To actually get to the gear, you have to remove the two screws indicated in the photo to separate the two tower halves.
This is what you get when you remove the two halves from each other. You can see the original "high" gear in the tower half just below the new "low" gear.
This next photo shows them side by side. The "low" gear has one extra winding on it as compared to the "high" gear. That's it!
There are two small brass bushings that need to be removed from the old gear. The harder part is getting the universal joint to be removed. I used a small jewelers screwdriver that just fit the slit in the plastic joint piece. That gave me a good, safe grip on it and then I just rotated it while also pulling on it. Eventually it came off. It is then just a matter of replacing the two brass bushings on the "low" gear and re-installing the plastic joint. I pushed it in until the end of the axle was flush with the inside of the plastic joint piece's hole.
If you look carefully at the brass bushings, you will noticed that its outside is round, except for one edge, which is flat. The two tower halves insertion slots have one side curved and one side flat. Insert the new gear and manipulate the bushings so that the flat side is down on the tower half. I put all the various plastic gears on that one half, which makes it easier to mate the other tower half to it. It is then just a matter of using the two screws to connect the two halves, and then re-assemble the entire engine. When run with my other A unit, both now ran at a somewhat similar speed. However, the trade-off I found was that the new gearing is incredibly loud. Your mileage may vary, but my final decision was to just stop messing with this second A unit, and I removed all the gearing from it. It is now just a dummy unit.