So I am making a car and need a way to measure speed of the car, however I can't merely count rotations of the wheel per second because the wheels have pretty high slippage. Is there any other way I could measure the speed?
Measure the time it takes to travel between two fixed points, then you will also need to consider acceleration and deceleration.
If there is high slipage on your traction surface, then you may want to reexamine your design. Autos usually use a vehicle speed sensor on the output shaft of the transmission. If you have non-driven wheels then rotation of one of those with tire diameter can be used to calculate distance, and distance / time = speed.
Tractortrailers have a hub mounted odometer which tracks their mileage. That requires knowing tire size.
Gps is not a fully accurate speed sensor because of the variation in points recorded for a given situation. For instance most GPS algorithms have a tendency to continue in a given direction if they have poor signal and it's possible for them to read wrong if they are working off of interpolated data. Also GPS speed is not necessarily over ground speed based on local interference. Consumer GPS has an error of about 3m and if you were to stand still and watch the output of raw gps data you would be anywhere within that spot if only a few sats were visible. Because GPS will work with as few as four satellites for a 'valid' 3D lock you could get into a situation where a change in velocity would not be recorded.
To see this situation in action stand still in an area with tall skyscrapers or heavy tree leaf cover. Both the metal in the buildings (reflected signal) and the water in the leaves (signal attenuation) prevent accurate position reporting.