**Work Time**

# Tire Weight: Part 1

By measuring the amount of surface area each tire presses into the ground and the pressure inside each tire, you can calculate the weight of a car.

You need to measure the surface area of the bottom of each tire. Make sure you get permission from the car’s owner before you take any measurements, and take measurements only when there is an adult supervising you.

- Most tire “footprints” will be roughly rectangular in shape. To find the length and the width of this footprint, use a piece of cardboard, a piece of string, or a yardstick to measure each tire’s footprint boundary. Measure the two sides that are easiest for you to reach.
- Record your measurements in inches.
- Ask an adult to use a tire gauge to measure the internal air pressure of the tire, or use the average tire pressure of most tires: 35.
- Find the amount of tire surface area touching the ground. To find this, multiply the length and width of the footprint. Your answer should be in square inches.
To find the amount of weight the tire is holding, multiply the surface area by the pressure (in PSI) of that tire. For example:

$28\text{sq}\text{.in}\text{.}\u2022\text{30}\frac{\text{lbs}}{\text{sq}\text{.in}\text{.}}=840\text{lbs}$Repeat these steps for each of the other three tires if you have time. If you do not have time, use the same measurement you took for the first tire to represent all 4 tires.

# Tire Weight: Part 2

- Add the weights together for all four tires to get the total weight of the car.
- To see how close you came to the real weight of the car, ask an adult to check the owner’s manual or look at the specification plate on the inside of the driver’s side door.

If you do not have access to an owners manual here are some sample weights of typical cars:

- A small car weighs about 3,000 lb.
- A medium car weighs about 4,000 lb.
- A small truck or SUV weighs about 4,500 lb.
- A large truck or SUV weighs about 5,500 lb.