Auto and Hardtop

Auto noun - A self-propelled passenger vehicle on four wheels.

Hardtop and auto are semantically related In some cases you can use "Hardtop" instead a noun "Auto".

Hardtop noun - A car that resembles a convertible but has a fixed rigid top.

Auto and hardtop are semantically related in car topic. You can use "Auto" instead a noun "Hardtop", if it concerns topics such as vehicle driven on streets.