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Speedometer Stopped Working: How Speedometers Work and What Causes Them to Stop Working

how speedometer works
Photo by Allen Skyy on Flickr / CC BY-ND 2.0

Whether you’re cruising down the freeway or slithering in a traffic jam, few things are easier than glancing down at your speedometer to find out how fast you’re going. Knowing your speed is one of the most basic and important pieces of information you need to drive safely. If your speedometer stopped working, you can’t be sure how fast you’re going. Have you ever wondered how your speedometer knows the exact speed?

Understanding the technology behind this important instrument can give you a greater appreciation for the humble speedometer.

What Is a Speedometer?

Speedometers tell you the speed of your vehicle.
Pixabay.com / CC0 1.0.

As its name implies, a speedometer is piece of equipment that tells you the speed of your vehicle. In most vehicles, it’s a circular gauge located prominently on your dashboard, with numbers arranged around the gauge. The smallest numbers are at the bottom of the gauge and they increase steadily as they climb the circle. A needle radiates out from the center of the gauge and points to the number that matches your current speed. Most speedometers actually have two rows of numbers, indicating the speed in both miles per hour and kilometers per hour.

A Short History of the Speedometer

The earliest cars didn’t actually have speedometers. This wasn’t a problem at first; most of the earliest cars topped out at relatively low speeds, so it was easy for their drivers to keep their speed under control. By the turn of the 20th century, however, top speeds had increased to around 30 mph, leading to an increase in serious accidents. In response, Otto Schulze invented the first speedometer in 1902.

These first speedometers were expensive and difficult to find, but by 1910 carmakers began to offer speedometers as standard equipment.

DID YOU KNOW: Early speedometers actually had two gauges: one for the driver, which was located on the dashboard, and a larger one at the front side of the vehicle. The larger, exterior gauge was there so the police could read your travelling speed.

Types of Speedometers: Mechanical and Electronic Speedometers

There are two types of speedometers: Mechanical and electronic ones. Pixabay.com / CC0 1.0.
Pixabay.com / CC0 1.0.

Broadly speaking, there are two types of speedometers: mechanical and electronic. Mechanical speedometers have been around since 1902 and are based on the designs of Otto Schulze. Electronic speedometers are a relatively recent invention and appeared during the 90s.

Now, let’s get back to mechanical speedometers to explain how exactly they work.

Mechanical Speedometers use Magnets to Display Your Travelling Speed

They are often called eddy-current speedometers because they use magnetic eddy currents to display the speed of your car. Mechanical speedometers are analog devices that attach directly to the transmission shaft of the vehicle. They gave drivers a reliable way to measure their speed at a time when electronic sensors didn’t yet exist.

They consist of several parts:

  • the drive cable
  • the mandrel
  • a spiral gear
  • permanent magnet
  • speedcup
  • hairspring
  • needle

The drive cable is wrapped around the transmission shaft and holds the mandrel inside. As the shaft rotates, the mandrel starts to rotate as well. The other end of the drive cable is attached to a spiral gear. This gear rotates with the mandrel, and is attached to the permanent magnet, which itself rests within the speedcup. As the magnet rotates, it creates a rotating magnetic field. This field then creates a drag force that pulls on the speedometer needle.

This pull is balanced out by the hairspring, causing the deflection of the speedometer needle to equal the speed of the vehicle.

DID YOU KNOW: Because speed, distance traveled and engine rotation can all be derived from the rotation of the transmission shaft, the drive cable is also linked to the odometer and the tachometer.

Electronic Speedometers rely on the Vehicle’s Speed Sensor to Display Travelling Speed

Electronic speedometers use a vehicle speed sensor rather than a drive cable to compute the speed of the vehicle. The sensor consists of a toothed metal disk, a stationary detector and a magnetic coil.

The disk is attached to the transmission shaft of the vehicle; when the shaft rotates, the teeth on the disk interrupt the magnetic field of the coil. The interruption triggers the detector and sends a pulse to the vehicle’s computer. The computer then uses these pulses to compute the speed of the vehicle, the distance the vehicle has traveled and how fast the engine is rotating. The vehicle’s speed is then displayed on a traditional analog dial or on a digital display.

Making Large Changes to Your Vehicle can Alter the Accuracy of Your Speedometer

Car tire size impacts the accuracy of the speedometer.
Pixabay.com / CC0 1.0.

For Example, going from a 21-inch tire to a 24-inch tire can throw off the accuracy of your speedometer by as much as 15 percent. If your speedometer is no longer accurate, you may need to recalibrate your speedometer. This is usually accomplished by using a powerful electromagnet to alter the field strength of the magnet in your speedcup, and this is something you shouldn’t try doing yourself.

Speedometers are not Completely Accurate, but they are Accurate Enough

Whether your speedometer is electronic or mechanical, it needs to be carefully calibrated to accurately translate the rotation of the transmission shaft into the speed of the vehicle. The default calibration of your speedometer relies on several assumptions about your vehicle, including the gear ratio in your differential and your vehicle’s tire size. Manufacturers perform extensive testing to determine the relationship of these factors to your vehicle’s speed and motion. Using that testing, they set the strength of the magnetic field and the resistance offered by the hairspring to match with the actual speed of the vehicle.

Speedometers are not completely accurate, but they’re usually only off by a few percentage points. A reasonably accurate speedometer is sufficient for most drivers’ needs; in most cases, it doesn’t matter whether you’re going 59.6 mph or 60.4 mph as long as you know that you’re going about 60 mph.

Speedometer Stopped Working? These are the Most Common Causes

Speedometer stopped working and stays at 0MPH.
44/52 – Speedometer by Scott Akerman on Flickr / CC BY 2.0.

If your speedometer stopped working and stays at 0 MPH, you should have your car checked ASAP. The most common causes of a speedometer that stopped working include are a faulty speed sensor, a broken gear on the speedometer, damaged wiring, or a faulty engine control unit.

DID YOU KNOW: Sometimes you might have an issue where the speedometer is working, but odometer is not. The reason for this are the odometer gears that are most likely broken, so you will need to replace them.

The Future of Speedometers

One of the biggest disadvantages of the current speedometer design is its location; looking down at the speedometer forces the driver to take his or her eyes off the road, if only for a moment. When you’re traveling at 60 mph, however, a moment is all it takes for an accident to happen. Future speedometers will be integrated into a heads-up display, or HUD, directly in the driver’s view. Other technologies, including lidar, radar and GPS, may eventually take the place of transmission-mounted sensors to compute the speed of the vehicle.

The speedometer, though not particularly glamorous, serves an important role in your vehicle. Being able to quickly and accurately know your vehicle’s speed is critical to help you navigate the roads safely. If it seems that your speedometer is not accurate or if the speedometer stopped working, have your car checked to determine the cause and fix it.