what's the meaning of "ground down"?

Dorothy is driving a car with a manual transmission. In the process of manually changing gears, probably because she knew she had to slow down when driving around the pothole, her timing was off.

When changing gears on a manual transmission car, one has to push in the clutch pedal and at the same time move the manual shift to it's respective position. If you start moving the shift too early or release the clutch pedal too early then you will 'grind the gears', a loudish event where the spinning gears of the transmission and the moving gears of the car's motor end up not syncing together in the correct timing. The noise of a ground gear comes when the spinning teeth of both sets of gears collide and scrape together rather then mesh together seamlessly the way they are supposed to do if the timing of the act is done correctly.

My bet, though I certainly could be wrong, is that this is some old text describing a time from the days before transmissions had synchronizers.

By using a file, grinding stone or some other abrasive, we 'grind down' the surface of another material. But I don't believe that is the meaning/intent here, though repeated bad shifting can 'grind down' gears.

I believe that 'ground down' could/should be here glossed as 'shifted down', eg. shifted down to second gear [from third gear]

You mean, Dorothy didn't know how to double-clutch?

There's not near enough context to determine that, Rog.

Thanks！

I think stepan is the closest. Dorothy probably shifted down into first, which is the hardest downshift, even with synchromesh, if you're moving beyond a couple miles an hour, and didn't do it very well, and ground the gears. Good on Dorothy for noticing the pothole. I missed one obscured by the low angle of the setting sun a couple years ago, and it was just the right size for the entire right front wheel to fall into it, destroying the tire and ruining the rim, and to cause the premature failure of the right front strut a few months later. Fix those goddamned potholes, local government, that's what I say.

almost nobody knows how to double clutch these days, roger. when you're really good and really familiar with a car, you can upshift or downshift without using the clutch at all. Old bugs were easy to do it in, and I had a couple RX7s I could do it in too.

If Dorothy was doing a couple miles an hour, MJ, then the pothole would present no problem. And she `navigated a deep pothole`, something that would hardly be worth mentioning in a story line if she was at first gear speeds.

ground down means to crush, pulverize, or reduce to powder by friction, especially by rubbing between two hard surfaces or to shape, sharpen, or refine with friction.

Yeah, like what Dorothy did to her gears as she unartfully down shifted.

Not in this particular situation it doesn't, Jessica.

"Grinding of the gears" is a common idiom.
Usually it means that the engines or gearboxes are old (or overtaxed), as well as the drivers being inexpert.

Fair enough.