Your code seems strange.
You need to read the analog voltage from each of the three directions. This gives
you a number between 0 and 1023.
You convert this to a voltage.
You convert this to a G reading according to the stated zero point and sensitivity of the device.
You then have a g force in each of the three directions of the device. You should see 1 g down,
which is gravity. You have to disregard this if you are calculating the acceleration of the car.