辆
liàng
If it rolls on wheels, it's 辆
The picture One thing rolling along the ground on wheels. No wheels, no 辆. 辆 is one thing rolling along the ground on wheels; 架 is one thing lifted off the ground into the air.
Where the picture shows up in 辆
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one at the gate
门口停着一辆车。
ménkǒu tíng zhe yí liàng chē
There's a car parked out front.
A car standing at the gate with all four wheels on the ground.
Trains take 列, planes 架, boats 艘. Only wheeled things take 辆.
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one you want to buy
我想买一辆自行车。
wǒ xiǎng mǎi yí liàng zìxíngchē
I want to buy a bicycle.
Spinning the front wheel of a bicycle parked outside the shop.
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the next one you're waiting for
下一辆公交车几点来？
xià yí liàng gōngjiāochē jǐ diǎn lái
What time does the next bus come?
Craning your neck at the stop as a bus rolls toward you in the distance.
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two coming through
前面有两辆救护车。
qiánmiàn yǒu liǎng liàng jiùhùchē
There are two ambulances up ahead.
Two ambulances rolling past in the lane ahead, one behind the other.
All of it in one picture
The car at the gate stands on its wheels, the bicycle you want rolls on its wheels, the bus you wait for comes rolling in, and the ambulances ahead run on wheels too. 辆 does not ask whether it carries people or goods; it asks whether it rolls along the ground on wheels. So anything counted with 辆 brings wheels touching ground with it. That is exactly where planes split off to 架 and boats to 艘.