An inclined plane is a simple way to measure the speed of a toy car. The basic idea is to measure the time it takes for the car to travel a known distance down the plane. You can then use this time to calculate the car's speed.

Here are the steps on how to measure a toy car's speed with an inclined plane:

1. Set up the inclined plane. You can use a variety of materials to create an inclined plane, such as a piece of wood, a book, or a stack of blocks. The steeper the plane, the faster the car will accelerate.

2. Measure the distance. Measure the length of the inclined plane from the starting point to the bottom.

3. Release the car. Place the car at the top of the inclined plane and release it.

4. Measure the time. Use a stopwatch or a timer to measure the time it takes for the car to travel the length of the inclined plane.

5. Calculate the speed. Once you have the time and the distance, you can calculate the car's speed using the following formula:

```

Speed = Distance / Time

```

The speed will be in units of distance per time, such as meters per second (m/s).