Why the Google Maps and Waze Speed Is Different from the One on the Dashboard

For those who’ve never used these apps before, the whole thing probably sounds as awkward as it can get. Why should anyone use the speedometer in a mobile app when the current speed is already on the dashboard, right?

This is a legit question, and there’s no easy answer to it. These apps, however, come with several extras, such as a warning when you’re going over the speed limit, so in theory, seeing how fast you’re driving right on the screen clearly comes in handy.

But on the other hand, those who’ve used these apps and relied on the speedometer and the shown limit probably noticed something that caused even more confusion. More often than not, the speed displayed by Google Maps and Waze is different than the one you see on the dashboard, so in theory, the speed limit warning could be inaccurate as well.

Only that, it isn’t, as the current speed information in navigation apps comes with higher accuracy than the readings you see on your dashboard.

And you’ll understand why in the next few paragraphs.

First and foremost, it all comes down to how each system (the mobile navigation app and the speedometer on your dashboard) determines the current speed of the vehicle.

Waze, Google Maps, and all their alternatives are based on a very simple concept that many of us learned about in elementary school: speed equals distance divided by time.

And because these apps use GPS positioning for the whole thing with a super-high accuracy (dual-GPS mobile phones, for example, can determine your location with an accuracy of up to 4.9 meters / 16 feet), you can imagine everything else is pure math. So in theory, the speed indicated by these mobile apps is very likely to be the current one for your vehicle.

On the other hand, speedometers installed on cars use a different approach. To determine the speed, for example, some carmakers use a rather simple algorithm that determines the rotation of one of the tires, with the information then sent to a sensor, then directed to the CAN system where it’s processed, and eventually displayed on your dashboard using either an analog or a digital indicator.

The speed reading, on the other hand, can vary depending on factors like tire dimensions, wear, and even the quality of the surface you’re driving on. So reaching a 100 percent calibration of the sensors is impossible because of all these variable factors, and this is why some carmakers turn to a clever trick that does lead to a small offset but which maintains the accuracy in an acceptable range.

For example, to display the current speed, some cars measure the actual speed, then multiply this value by a defined speed factor and add between 2 and 4 kph (1-2 mph). This should eventually cause an industry-accepted accuracy of 10% for the displayed speed reading, which in theory means that the difference at city cruising speeds shouldn’t be higher than 8 or 9 kph (4-5 mph).

In other words, the value you see on the dashboard is generally higher than your actual speed, so the aforementioned approach is eventually a double win: not only manufacturers can maintain the accuracy of their readings close to the real values, but drivers end up going slower too. Some people believe carmakers do this on purpose, as they want people to drive slower, but as explained above, this isn't the case, as perfect calibration simply can't be obtained with all these variable factors.

So at the end of the day, the current speed displayed by navigation apps is essentially the one that’s more accurate than what you see on the dashboard.

Of course, this doesn’t necessarily mean it should replace the car’s speedometer, but if it sends a notification warning you’re going too fast, then you’re definitely doing it.