What is the accuracy of your car speedometer?

"If my seated navigator shows that my speed is 4/5 miles slower than what the car speedometer says, which one do I believe?" from

  – There are car owners everywhere.

In recent years, many drivers have reported this exact phenomenon, as satellite navigation has become more common in automobiles, and this situation is actually common to almost all cars on the market. But why is this happening? Of course, can automakers make their speedos accurate to the exact mph or km/h you are driving?

How does a car speedometer work?

Speed ​​is a measure of the distance that varies over time. However, the car speedometer does not actually measure your speed from point A to point B. Car Speedos usually work by measuring the car's drive shaft, axle or wheel rotation. Then they use some basic math to infer the rotation and determine how fast you are going. It is very similar to the concept of a bicycle speedometer.

However, if the diameter of the wheel/tire changes, the extrapolation calculation will be incorrect. For example, if you put a new tire on your car [more treads, it will wear thousands of miles], the diameter will increase or the tire pressure will increase. This means that every time the wheel turns, the car will travel farther, which means you are faster.

If the diameter is reduced [for example, worn tires, less air in the tires, different brands of tires with slightly different sizes, more load in the car, the weight will drop and the tires will be compressed], then every turn of the car Driving speed will be slower

Error range in car speedometer

In these cases, the wheel diameter difference may be small [perhaps a few millimeters], but at 30 mph, your wheel spins 6-7 times per second, so it can quickly produce a few miles per hour. This margin of error should be considered when applicable law and how manufacturers calibrate vehicle speed.

How satellite navigators work

Satnav uses GPS satellite tracking to measure your speed over time based on actual distance. It repeatedly locates your exact location on Earth through the satellite and calculates how far you have gone and then divides the time it takes you to walk that distance. The accuracy of satellite navigation depends on the quality of the satellite signal and is not affected by the car tires. Many satellite navigation cannot explain vertical changes, so if you are driving up and down a steep hill, it may not be accurate. Over time, larger distances reduce rounding errors, so they are inherently more accurate at higher speeds.

Some factory satellite navigation systems will also use data from cars to integrate with GPS signals to improve overall accuracy.

British car speedometer

British law is based on EU standards, but with minor changes. Speed ​​cars must never show below actual speed and must never display more than 110% + 6.25 mph of actual speed. So if your actual speed is 40mph, your speedo can legally reach 50.25mph readings, but not below 40mph. Or to put it another way, if your speedo reads 50mph, your speed won't exceed 50mph, but in reality you might only be driving at 40mph.

To ensure that they comply with the law and ensure that the display of the speedometer is never lower than the actual speed, car manufacturers often deliberately calibrate their Speedos to "high." A certain amount, therefore, this is why your speedometer reading is higher than the satellite navigator.