counter for large vehicles
輛 centers on counting large wheeled vehicles. Its meaning is specialized to that counter function.
輛 combines 車 (vehicle) with 兩, which likely contributes the sound and originally meant a pair, suggesting a vehicle with two wheels or a pair of vehicles.
The left side is a vehicle 車, and the right side 兩 looks like a pair of wheels or a double structure. Picture counting large vehicles by their pairs of wheels.
For リョウ, imagine a row of large vehicles lined up: row -> リョウ, and you count them one by one.
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counter for carriages (e.g. in a train); counter for vehicles