Why Your Speedometer Reading Is Deceiving You?

If you’re keen when driving, you may have noticed that the speed on your car’s speedometer doesn’t always align with the one on your phone’s GPS. 

Question is, which device is showing accurate information and which one isn’t? Do speedometers show true speed? 

If you’ve had these questions lingering in your mind, your suspicions are warranted.

The truth is, a car’s speedometer rarely, if ever, shows the actual speed.

In the following post, we’ll explain just how accurate (or inaccurate) speedometers are and the factors that play a role in this. Let’s dig in.

Are speedometers 100% accurate?

Like most car owners, I’d like to believe that the speedometer always shows an accurate reading of the speed I’m traveling at. 

Unfortunately, this is not the case. Speedometers are never 100% accurate. You can easily confirm this by looking at the speed displayed on your phone’s sat-nav app. 

Your car’s speedometer accuracy depends partly on where you come from. In the U.K., for instance, speedometers are designed to not read. This means the speedo’s reading will never be lower than your car’s true speed. 

Meanwhile, in the U.S., a speedometer can over-read or under-read but only by a small margin. 

Regardless of where you come from, the speedometer reading should be fairly accurate. 

However, don’t expect it to have the same precision as your phone’s GPS which relies on satellites to show your exact position.  

How does a car speedometer work?

Speed is calculated by estimating the distance covered over a given timeframe. 

Interestingly, car speedometers don’t work by estimating the length of time you spend getting from point A to point B. Rather, they determine speed by gauging how fast the wheels are rotating. 

You’re probably familiar with state-of-the-art features incorporated in modern cars. These include Electronic Stability Control (ESC) and anti-lock braking (ABS). 

What you don’t know is that each of these mechanisms relies on speed sensors positioned on individual wheels. 

An element known as the engine control unit (ECU) estimates the number of rotations made by each wheel. 

By applying some basic maths, the ECU converts the speed of the rotations to the car’s linear speed. This figure is what’s displayed on your speedometer. 

How do my tires impact the accuracy of my car speedometer?

Given that the speedometer relies on the wheels’ rotations to determine speed, it is no wonder that changing the wheel diameter impacts speed. 

When you purchase a new car, its speedometer is calibrated by the manufacturer according to the size of the intended wheels. 

If you switch to new tyres that have a different diameter, the circumference or rotation automatically changes. 

For instance, buying bigger tyres means the diameter increases; hence, allowing you to cover a larger distance with each rotation. The same concept applies when you increase the tyre pressure. 

Now, unless the speedometer is re-calibrated to account for the change in wheel size or air pressure, it will end up giving a false reading. 

It might show that you’re driving at 60 miles per hour when you’re probably traveling at higher speeds of 63 mph or more. 

If the situation was reversed whereby you switch to smaller tyres or reduce tyre pressure, this means the diameter decreases. 

As a result, each of the car’s tyres covers a shorter distance with each rotation. This then means that the car’s true speed will be slower. 

Does the speedometer read higher than actual speed?

This depends on the car model and whether or not you’ve changed the tires. 

As explained earlier, switching to smaller tyres would automatically decrease the distance covered per rotation. 

So in reality, you might be driving at a speed slower than what’s displayed on the speedometer. 

Margin of error in a car speedometer

To avoid massive discrepancies in speed readings, car manufacturers account for these changes when calibrating their speedometers at the factory. 

Most automakers deliberately calibrate their speedometers to show speeds higher than the true speed. 

In fact, the majority of speedometers are designed to operate with a 5% margin of error. 

So if you want to get the actual speed, you’re better off relying on sat navs rather than the speedometers calibrated by car manufacturers. 

Sat navs rely on gps satellite tracking to determine the specific distance covered within a given period. 

Since their mode of operation isn’t affected by changes in your car’s tyres, they have a higher overall accuracy. 

The only thing that can affect their precision is the satellite signal quality. Generally, the stronger the GPS signal, the more accurate the speed reading will be. 

Best way to troubleshoot an off-speedometer?

It’s pretty easy to confirm whether your car’s speed is precise or not. 

The easiest way to go about this is to compare its reading to the one shown on your phone’s sat nav. 

If it’s not the same, then you’ll know that your speedometer is off.

Another approach is to test your vehicle’s speed on a highway. You’ll need a 5-mile stretch of flat motorway with mile markers and a speed limit of at least 60 miles per hour. 

You’ll also need help from a friend or family member, who can record your time with a stopwatch while you concentrate on driving. 

The goal here is to determine the length of time it takes you to cover the predetermined distance. For instance, if you’re traveling at a speed of 60 miles per hour, you should spend exactly one minute to cover a mile.  

Ask your assistant to start and stop the stopwatch while you drive to cover the preset distance. 

Try as much as possible to maintain a constant speed. If your vehicle has cruise control, consider using it to maintain a steady speed. 

How do I work out my speedometer’s true reading?

For illustration purposes, let’s say you spent 5 minutes 30 seconds to cover the entire 5-mile stretch. 

To work out your car’s speedometer speed, start by converting this time into the same metric. 

To convert it to minutes, simply divide the second’s portion by 60.

So the total time will be: 5 + 30/60 = 5.5 minutes. 

Next, insert this figure into the equation below:

Actual speed (in MPH) = 60 / minutes * number of miles covered

If the speedo shows a higher speed than the answer you get, it means the speedo is too fast. And, if the speedo’s reading is lower than the result, this means the speedometer is slow. 

From the example above, your car’s speed is 60/ 5.5 * 5 = 54.5 MPH

If the reading on the speedo doesn’t match, this means your car speedometer’s accuracy is a little off. 

Conclusion

If you’ve been wondering whether your car’s speedometer is accurate, there’s a high probability that it doesn’t. 

This is because a speedometer doesn’t function in the same way a satellite navigation unit does. 

The latter pinpoints your precise location and estimates the duration taken to cover a specific distance, resulting in maximum accuracy. 

A speedometer, however, gauges the number of rotations car tyres make to determine speed. This approach reduces its accuracy by a small extent.