Car appreciation... the aesthetics, the engineering, etc

Not sure if serious; however:

Testing whilst stationary won't tell you much because what you really want to know is how much you can hear above road / wind / engine noise.

You'll need a reasonably prominent upper frequently range that avoids being harsh and unpleasant. Female vocal, orchestral strings, acoustic guitar can test this well.

You'll want a punchy bass, dynamics arguably more important than getting very low frequency. Just pump some 90s techno for this.

Then try radio 4.