Some speedometers in older cars are analog and do not show precise speed very well. That is where a GPS-guided speedometer built into a mapping app come in to make sure the driver isn't unknowingly exceeding the speed limits and falling for speed traps.
I disagree as to the accuracy of navigation app speeds, they're usually incredibly accurate (in my experience) because they combine GPS and accelerometer data from the device. The software you're using by Google might need tweaking or your phone may have taken damage from a fall affecting the accelerometer readings, but on modern phones apps like Waze are incredibly good at accurately depicting speed.
And since many people across the world drive older cars with analog displays, it's also worth noting that most car manufacturers overstate displayed speeds by ~3-5% as most countries mandate this. That means if you're passing a speed trap at highway speed and are left to estimate your speed on an analog speedometer, perhaps with non-factory sized wheels which further inflate displayed speed on your car's speedometer, you could freak yourself out very often because you don't know what your real speed is when passing a speed trap. Hence why I wish Apple add an optional speedometer in Apple Maps. Nobody wants this stress.
