We all know what speedometers do; there’s no need to even have that conversation. But most people just take the speedometer for granted. We use it every day, but we don’t often consider exactly how it works. That is, we don’t consider it until one breaks. When the speedometer starts giving us error, or stops responding entirely, it can create an enormous problem for drivers, especially in conditions where you need to keep a certain speed. Speedometer errors can often be small and insidious enough (say, 5 mph off target) that we don’t really notice them, but even 5 miles per hour can be enough to be the difference between being pulled over or no.
In order to introduce speedometer gears, it’s important to know how speedometers work, and how the gears control and influence their performance. Most of today’s cars have mechanical speedometers, which is the type of speedometer that uses gear. The other type is called an electronic, or digital, speedometer, and it works differently. An electronic speedometer actually uses a sensor attached to the transmission to determine the speed of the car. There are no gears involved at all, and tuning it is a simple matter of changing the settings.
However, electronic speedometers were only invented in 1993, and aren’t required by law, so they haven’t had a great amount of time to catch on to the general car population, especially since they aren’t any cheaper for manufacturers. Until the day comes where they become cheaper or more popular, most cars will continue to have mechanical speedometers. For the most part, these work just as well. They’re proven to be just as accurate, and they have been around since 1902, so many mechanics and manufacturers are just more comfortable with them as devices. Also, they can actually be cheaper than electronic speedometers, which is probably the main incentive for their existence at the moment.
Mechanical (or eddy-current) speedometers are, predictably, much more simple devices. Like the electronic speedometer, the mechanical speedometer measures either transmission or (rarely) the tires – the rotation of these two pieces of the car are the only reliable indicators of speed. The actual dashboard speedometer connects directly to the drive cable, which is a flexible cord that goes straight from the dashboard to the transmission, where the speedometer gear is located. From the gear, the drive cable also leads to the speed cup and the magnet that reads the transmission.
The speedometer gears are the part that calibrates the signal the magnet sends, so to speak. Basically, the magnet reads how fast the transmission gears are turning, and the gears are calibrated to translate this information into data that we can recognize easily. Usually if there’s a problem with the speedometer, this gear is pretty durable, and it’s the drive cable that is the problem. But there are various reasons why the speedometer gear would have problems. They are very precise devices, so even the smallest amount of mistake can do pretty heavy damage to a speedometer’s ability to show your speed correctly.
A speedometer gear has to be calibrated on only to your transmission speed, but to its own height and the size of the other devices within the transmission. If it’s attached to your tires, then it also has to calibrate for the size of those tires – so if you change your tires, you may have to buy a new speedometer gear to have it calibrated correctly. Even if your speedometer gear is worn down by general use (usually happens after about 10-15 years of driving), you’ll probably need to get a new one. They can be calibrated at home, even by somebody with a pretty novice understanding of car gears, but it’s a complex formula for which you’ll need to find an equation that’s specifically made for your vehicle.