Industrial Outpost - The Official News Source of PSC

Petroleum Product of the Week: Speedometer

One of the many things I never really thought about the workings of before includes the speedometer. I mean, I pay attention to it. I always keep an eye on it to make sure I’m not speeding or anything! But I have no idea how it works.

Surprise! It’s a lot more complicated than I would have imagined.

And petroleum is there to make sure that you’re well aware of how fast you were going.

Mechanical Workings of the Speedometer

So, your speed is calculated by dividing the distance you travel by the time you take to arrive somewhere. Your speedometer does all that math for you so that you can focus your eyes on the road with a quick glance down to check yourself every now and then.

The driving force behind how this all works is something called electromagnetism. My high school brain hurts just reading that word, but it’s not as bad as it sounds. Essentially, it’s a bunch of magnets working together inside of this mechanism to determine your speed.

A cable connects the wheels to the speedometer you see in your car. This cable is flexible and works like a driveshaft—rotation on one end causes rotation on the other end. This is where that electromagnetism comes into play.

When that cable rotates, a magnet inside the speedometer case also rotates at the same speed. The magnet is located inside of a part called the speed cup, but they are not connected—air separates them. The speed cup is attached to the pointer that you see measuring your speed.

That magnet creates a magnetic field in the speed cup which works as an electricity generator. The dial rates, trying to match the energy created by the magnet, but it’s limited in its range because of a hair spring that tightens while this is all happening. Still, it rotates and that rotation shows you the speed in which you are traveling.

The Sum of Its Parts

Now we’re diving into the petroleum aspect of the speedometer. Most of the parts are made of plastic, which is derived from petroleum.

Specifically, 40% of speedometer mechanical parts in recent years were made from plastic polymers. Even better yet, so many of the newer, electronic models are nearly entirely made from plastic.

Many components of the speedometer are typically made from nylon, including the case, worm drive, and magnet shaft. However, in recent years the case has been made from polybutylene terephthalate (PBT) polyester, which is more water-resistant.

The glass lens covering your speedometer is actually not made from glass, but typically from a clear polycarbonate. This is a strong and flexible plastic that holds up to heat, impact, and moisture.

With the help of plastic parts—and some science—you’ve been armed with a tool that helps you check yourself. Now, be wise and drive safe!


Kayleigh DeMace

Kayleigh is a content writer with a BA in technical writing/literature and an MA in creative writing. A native of Northeastern, PA’s coal region, she is no stranger to the industrial world. When she’s not at work writing, she’s at home writing and reading, forever honing her craft and exercising her writing muscles. Her work has appeared on The Writing Cooperative and as an Honorable Mention in East Meets West American Writers Review.