Polar FIS+ uses the speed/rpms from the car to calculate the 0-60
Dynolicious uses the accelerometer/gyro in the Ipad/Iphone to calculate acceleration
VI Monitor does the same as the Polar FIS+
Racelogic VBox uses GPS and was proven to be very accurate when comparing to a £30k F1 datalogging GPS system.
Dynolicious is fairly accurate as long as you accelerate on one level, if the level changes it becomes more inaccurate.