Three dimensional GPS or vehicle position data is used to determine a slope the vehicle is traveling over at a specific point in time. The slope data can then be combined with other metrics to provide an accurate, slope corrected vehicle mass. The vehicle mass can then be used along with other vehicle data to determine an amount of work performed by a vehicle, enabling s detailed efficiency analysis of the vehicle to be performed. To calculate slope, horizontal ground speed (VHGS) can be calculated using the Pythagorean Theorem. One can take the Z/Up magnitude and divide it by the horizontal ground speed. Replacing Z, x and y with directional vectors enables one to calculate slope. The slope data is then used to determine the mass of the vehicle at that time. Pervious techniques to calculate mass did not factor in slope, and thus are not accurate.