Object oriented Design: Scalable and maintainable car store system

I had an interview yesterday and had asked an OOD question:

Race-Car Store System:

The system stores information about cars available for players.

• Two types of gear changing strategies: manual/automatic.
• Two types of fuel: gasoline/diesel.

Design a system that can produce cars requested by players (If the player wants a car with manual gear changing and burn diesel, your system should provide one instance of the car meets the requirements). The system should have good scalability and maintainability.

My thoughts and solution:

My thought is the requirement contains two attributes: gear and fuel. I plan to make an abstract class contains the attributes and corresponding behaviors. Considering scalability would have an interface Movable which contains behaviors what a car can do.

If any new attribute is added in the future, either create a new abstract class contains the new attribute or add the attribute into the existing abstract class, if there's new behaviors are required, I would either create new interface or add the behavior into existing interface.

Here's what I have done: An interface contains general behaviors, currently has showSpecs() only.

public interface Movable {
    public String showSpecs();
}

An abstract class contains attributes fuel and gear

public abstract class Car implements Movable {
    String gear;
    String fuel;

    abstract void setFuel(String fuel);

    abstract String getFuel();

    abstract void setGear(String gear);

    abstract String getGear();
}

Now the race car class implementaion:

public class RaceCar extends Car {
    public RaceCar(String fuel, String gear) {
        this.fuel = fuel;
        this.gear = gear;
    }

    public void setFuel(String fuel) {
        this.fuel = fuel;
    }

    public String getFuel() {
        return this.fuel;
    }

    public void setGear(String gear) {
        this.gear = gear;
    }

    public String getGear() {
        return this.gear;
    }

    public String showSpecs() {
        StringBuilder sb = new StringBuilder();
        sb.append("Gear:").append(this.gear);
        sb.append("Fuel:").append(this.fuel);
        return sb.toString();
    }
}

Below is the main class I have:

public class Main {
    public static void main(String[] args) {
        System.out.println("get started...");
        Car car = new RaceCar("diseal", "automatic");
        System.out.println(car.showSpecs());
    }
}

The interviewer replied that the solution I provided is not scalable and hard to maintain but didn't provide details so I am still confused about what mistakes I made and how to improve it.

Can anyone help share your thoughts and point out what am I supposed to improve?

Thanks!