Your car’s dashboard has a bunch of things to tell you about your car, like how much gas you have left, how fast you’re going, and if everything’s running smoothly. But one thing you’ll probably check often is the speedometer. The job of the speedometer is to indicate the speed of your car in kilometers per hour. Analog speedometers use a needle to point to a specific speed, which the driver reads as a number printed on a dial.
Back in the day, speedometers were pricey extras for cars. It wasn’t until around 1910 that car makers started putting them in every car as a standard feature. One of the early suppliers of speedometers was a company called Otto Schulze Autometer (OSA). They made their first speedometer in 1923, and its design stuck around for about 60 years without much change.
There are two kinds of speedometers: electronic and mechanical. This article will mostly talk about the mechanical ones because electronic speedometers are newer, with the first one showing up in 1993.
Mechanical Speedometer
In 1902, a German inventor named Otto Schulze patented the first eddy-current speedometer. Back then, cars were getting more common and faster. At the start of the 1900s, the average car could go about 30 miles per hour, which seems slow to us now but was super-fast compared to horse-drawn carriages. Because cars were speeding up, there were more serious accidents happening.
Schulze’s invention let drivers know exactly how fast they were going so they could drive safer. Around the same time, many countries started setting speed limits and making sure drivers followed them with police officers. One early solution was to have cars with speedometers that had two dials—one small one for the driver and a bigger one that police could see from far away.
To measure how fast a car is going, we need to track how fast either the wheels or the transmission are spinning and show that on a gauge. In most cars, this measurement happens in the transmission. The drive cable is a key part of this process. It’s made up of tightly wound coil springs wrapped around a center wire. This design makes the cable very flexible so it can bend without breaking, which is important because it needs to wind its way from the transmission to the speedometer in the instrument cluster. The cable is connected to gears in the transmission. When the vehicle moves, these gears turn the center wire inside the flexible cable. This wire then sends the speed information along the cable to the speedometer, where the actual speed measurement occurs.
Every speedometer needs to be calibrated to ensure that the torque produced by the magnetic field accurately reflects the car’s speed. This calibration has to consider various factors, like the gear ratios in the drive cable, the final drive ratio in the differential, and the size of the tires. All these things affect how fast the vehicle goes. For instance, let’s talk about tire size. When an axle turns once, the tire it’s attached to also turns once. But a tire with a bigger diameter will travel a greater distance in one revolution compared to a tire with a smaller diameter. This is because the distance a tire covers in one revolution is its circumference.
Electronic Speedometer
An electronic speedometer doesn’t use a drive cable; instead, it gets its information from a vehicle speed sensor (VSS). This sensor is usually attached to the transmission output shaft or the crankshaft. It consists of a toothed metal disk and a stationary detector with a magnetic coil. When the teeth on the disk pass by the coil, they disrupt the magnetic field, creating a series of pulses. These pulses are sent to a computer. For every 40,000 pulses from the VSS, the trip and total odometers increase by one mile. The speed is also calculated based on how frequently these pulses occur. The car’s circuit electronics are designed to display the speed either on a digital screen or on a traditional analog system with a needle and dial.
Speedometer Accuracy
No speedometer can be 100 percent accurate, they usually have a margin of error. Most manufacturers design them to be within a certain range, typically around 1 percent to 5 percent too slow or too fast. If a car is kept in good shape according to the manufacturer’s specifications, its speedometer should stay within this range. However, if a car is modified, the speedometer might need to be adjusted.
One common modification that can throw off the accuracy of a speedometer is changing the size of the tires. Bigger tires cover more distance in one full rotation. Let’s look at an example.
Your car comes with tires that are 21.8 inches in diameter, so each tire covers 68.5 inches in one full rotation. Now, let’s say you want to swap them out for tires that are 24.6 inches in diameter. These new tires cover 77.3 inches in one rotation, which is nearly 10 inches more. This change significantly affects your speedometer. When it shows 60 km per hour, your car is actually going about 67.7 Km per hour, which is almost 13 percent faster.