Answer Geek: How a Car's Speedometer Works

<br> -- Q U E S T I O N: How do gauges on cars and aircraft work? The speedometer will still accurately measure speed regardless of how hard the engine works, such as a car going up or down a hill. How is that possible? How does an aircraft measure its speed? Does it measure wind flow over the wings? If so, how do instruments compensate for cross winds? And how does an attitude indicator function, despite any g-forces that act on the aircraft? — Drew U.

A N S W E R: Gosh, Drew. That’s an awful lot of questions. I count six question marks! The things is, I get paid by the column not the question, so that’s a pretty big load for one lone Answer Geek in a single week, especially given that there is a major holiday to consider and all. Not to mention the fact that when you sent in your question, you didn’t check the little box that would have let you make a small donation to yours truly for all the effort. But then nobody ever does.

On the other hand, those are some darn fine questions you’ve asked. If you think about it, there’s quite a bit at stake in the accuracy of all those dials and gauges. The ability to measure your rate of forward progress accurately is pretty key for both car and airplane safety.

For example, you always want your pilot to have a idea about just how fast the airplane is moving when you are coming in for a landing. And having an accurate reading of your vehicle’s speed is very helpful for avoiding those pesky traffic tickets. So here’s what I’m going to do, Drew. Conscientious Answer Geek that I am, I’m going to answer all your questions, but I’m going to divide them in half. This week we’ll look at cars; next week, airplanes.

Translating the Numbers

So let’s begin with the trusty old speedometer, shall we? The speed with which your car is traveling may indeed be related to how hard you engine is working, and whether you are going uphill or downhill, but those factors have nothing at all to do with measuring the rate of speed. Instead, the rate of rotation of a drive shaft is used to determine how fast your car is moving. The trick here is translating that rotation into its miles-per-hour equivalent.