Answer Geek: How a Car's Speedometer Works

-- 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.

Here’s how it is done in analog speedometers — the kind where a needle points to a number on a dial. On the drive shaft is a worm gear, which is threaded like a screw. That worm gear turns a notched gear which sits at one end of the speedometer cable. Inside that cable is a rotating wire which is driven directly by the rotation of the drive shaft.

Now at the other end of the wire inside the speedometer cable is a magnet. That magnet sits inside something called a drag cup, which is made of a non-magnetic metal, usually aluminum. The magnet spins as the wire rotates, creating a magnetic field that forces the drag cup to turn in the same direction. Attached to the drag cup is a spindle that connects to the pointer on your speedometer. A spiral spring on the spindle serves to counter the rotation of the drag cup.

If you accelerate, the increased magnetic field moves the drag cup further around; as you slow down, the spring tugs the pointer back. The whole system is calibrated so that the pointer indicates the proper speed. When you stop moving altogether, the spring pulls the pointer all the way to zero.

Digital Works

Digital speedometers are a little different. A car with a digital speedometer uses a speed sensor, which usually consists of a magnet surrounded by a wire coil, quite like the pickup on an electric guitar. The sensor is mounted directly next to a gear on the transmission, and as the gear spins, its teeth whiz by, interrupting the magnetic field on the sensor. The process is similar to the way a vibrating string activates a guitar pickup. The movement of the gear produces an electrical current that is directly proportional to the rotation speed of the gear. A computer chip processes the current and translates it into a number that corresponds to the speed at which you are traveling, in either miles or kilometers per hour.

Got it? Good. Next week we’ll take a look at how airspeed is determined. In preparation, Drew, you might want to bone up on Bernoulli’s equation, because as much as I hate to do it, I think I’m going to have to throw in a little math to make it all clear.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.