Hi,
you cannot get a speed signal from any part of the ESC, neither the PWM signal coming in from the receiver nor from the power lines from the ESC to the motor. What you then measure it throttle position, which is not relative to speed. Of course, if you keep the throttle still at a certain point, it will give a certain speed once stabilised, but it's not a speed reading.
Simple question, what car to you have? Please post make and model (or a picture). I have made a very simple speed reader a little while ago using a magnet on the wheel (just with a dab of hot glue) and a hall sensor. Using two sensors, or two magnets, can also give you direction.
But first, spec the car. Next, we can go into hardware. I think an Arduino Nano, two hall sensors, a magnet and two 4 digit 7 segment displays (TM1637) will get you going, but I need to see the car to know if we can fit it.
Fun project to learn, BTW.
Hugo