numerary adjunct for vehicles
輌 is a specialized counter for vehicles, used as a numerary adjunct when counting cars, carts, or similar wheeled conveyances.
輌 combines 車 (vehicle) with 両, which likely contributes the sound and the idea of a pair or set, fitting the counter sense. The exact historical development is uncertain.
A vehicle 車 paired with 両 (both, two) suggests counting vehicles in sets: 輌 is the counter for vehicles.
For リョウ, imagine a row of vehicles lined up for counting: row -> リョウ, and each vehicle is one 輌.
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Tokyu Car Corporation