MMA7455 How to interpreter the readings ?

Hello. I am trying to get the speed of a car from an accelerometer mounted in it . I will use the formula vf=vi +/- at to calculate the speed from acceleration. (vf is the speed at this moment,vi is the initial speed or the last vf read, a is the acceleration and t is the time between 2 readings). The problem is the that i don’t know how to interpreter the readings. In I2C the mma7455 will work in 8 or 10 bits? Will it be the g force that i selected /256(in 8 bit mode) or /1024 ( in 10 bit mode). Is 8 g equal to 80 m/s?

#include <Wire.h> //Include the Wire library
#include <MMA_7455.h> //Include the MMA_7455 library
#define calibratecicles 1000

MMA_7455 mySensor = MMA_7455(); //Make an instance of MMA_7455

char xVal, yVal, zVal; //Variables for the values from the sensor

void calibrate()
{
  float calx=0,caly=0,calz=0;
  for(int i=0;i<calibratecicles;i++)
  {
    xVal = mySensor.readAxis('x'); //Read out the 'x' Axis
    yVal = mySensor.readAxis('y'); //Read out the 'y' Axis
    zVal = mySensor.readAxis('z'); //Read out the 'z' Axis
    calx=calx + float(xVal); 
    caly =caly + float(yVal);
    calz=calz + float(zVal);
  }
  calx=calx/calibratecicles;
  caly=caly/calibratecicles;
  calz=calz/calibratecicles;
  /*if(calz>0)
  {calz=calz*(-1);
  } 
  else
  {
    calz=abs(calz);
  }
  */
  mySensor.calibrateOffset(abs(calx),abs(caly),0);
}
void setup()
{
  Serial.begin(9600);
  // Set the sensitivity you want to use
  // 2 = 2g, 4 = 4g, 8 = 8g
  mySensor.initSensitivity(8);
  // Calibrate the Offset, that values corespond in 
  // flat position to: xVal = -30, yVal = -20, zVal = +20
  // !!!Activate this after having the first values read out!!!
  //mySensor.calibrateOffset(5, 20, -68);
  calibrate();
}

void loop()
{
  xVal = mySensor.readAxis('x'); //Read out the 'x' Axis
  yVal = mySensor.readAxis('y'); //Read out the 'y' Axis
  zVal = mySensor.readAxis('z'); //Read out the 'z' Axis
  
  Serial.print(xVal, DEC);
  Serial.print("\t");
  Serial.print(yVal, DEC);
  Serial.print("\t");
  Serial.println(zVal, DEC);

}

char xVal, yVal, zVal; //Variables for the values from the sensor

This is odd. char ?? 10 bit value..

seen this ?
http://playground.arduino.cc/Main/MMA7455#.UxYX7uN5PoE

Thanks for your answer. I saw that link , but i followed and used this tutorial http://arduino-info.wikispaces.com/accelerometer-MMA7455 because it seemed easier to follow. From there i got the library. I did myself the calibration function. I will read more thoroughly the guide from playground and try that code.