Unique

Usually, it's not a good idea to calculate displacement by using accelerometer data because of many types of error that exist in acceleration signals. For a moving car, especially, GPS is the better option. But if you really need to use accelerometers, try to calibrate it first, and get velocity and displacement signals by integrating accelerations. You may need to do some signal processing to clean up your signals.

These are raw values that you get from the sensor. To get the actual acceleration values you have to apply the conversion from raw to processed data for acceleration. It's usually a constant division. For example, you have to divide the values by 856 to get the actual acceleration values (856 was just an example). You have to refer to your sensor's configurations and find that number. It is the acceleration sensitivity. The higher, the more sensitive it is!

Can you print out the average values of your acceleration signals for, say, 5 seconds of data collection? Simply, collect 5 seconds of signals and calculate the average values of each signal separately. I'm speculating that you need to subtract the values instead of adding them! But let us know the average values and then I'll be more sure!

The accelerometer's bias error is a very well-known problem. To fix it, just read its values for, say, 10 seconds. Calculate the average of the 10-second signal. This value is the bias value of your accelerometer. Before you calculate the velocity of the movement, subtract the bias error from the accelerometer's values. This should give you a cleaner velocity profile.