高級車って英語でなんて言うの？

If you want to use these phrases in a sentence, you can say:
"That is an expensive car."
"I can't afford a luxury car."
"I love luxury cars."

1 and 2 simply confirm the great cost of the car.
3. Is informal and indicates considerable expense.

"sports cars" is one of the most common ways to describe a fast, sleek, and expensive automobile.

Luxury (noun)
Used often to refer to things that are expensive

Example:
That is a luxury car
In this example 'luxury' lets us know that the car is expensive.

Luxurious (adjective)
Example
That is a luxurious car.
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Opulent (adjective)
This is used to refer to something that is costly and luxurious

Example
A: What is your dream car?
B: A Bentley
A: Those cars are quite opulent

"A lavish car"
This is an excessively expensive car.

"A posh car"
This is an informal way for describing something that is elegant or stylishly luxurious.
e.g He drives a posh car ,I have no idea how he was able to afford it.

"A car that costs an arm and a leg."
Body parts,particularly arms and legs are priceless entities we cannot attach monetary value to.If something costs and arm and a leg it means it is really valuable or expensive.You could also say :

-"I can see that car cost a pretty penny".
-"This car must be very pricey
-"The cost of that must must be quite steep".