The typical approach is to fuse GPS, inertial, and wheelspeed sensors to track a vehicle's motion in space. They all have their shortcomings, and all compensate for each other.

GPS is, as you said, wildly inaccurate, disconcertingly often. Wheelspeed is great when you're moving, but at low speed it tends not to have the granularity you want. Inertial sensors are pretty amazing nowadays, but they still drift.

To illustrate, if your wheels aren't moving, then you can ignore the IMU telling you that you're sinking into the ground at 5mm/s. Watching the monitor of a mapping vehicle as it depicts the car returning to mother Earth is pretty hysterical though. Until you have to fix it.

Actually, with a good receiver you can use GPS for very precise speed measurements by using the doppler shift of the carrier signal. It's just that most receivers don't give you access to that so the only other option is using the current and previous position and the timings of them.