Crossword Genius

In cars, they’re generally forward (5)

Ross

I believe the answer is:

gears

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This could be a double definition.

'in cars' is the first definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Perhaps there's an association between them I don't understand?

'generally forward' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'they're' is the link.

Can you help me to learn more?

(Other definitions for gears that I've seen before include "Sarge (anag.)" , "Rages (anag.)" .)

I've seen this clue in the Evening Standard.
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