With the goal of improving drivability, this research aimed to clarify the mechanism of vehicle longitudinal acceleration, focusing on tip-in acceleration. Conventional typical analysis methods include experimental modal and model-based analysis. However, since the former requires the measurement of impulses and other input forces while the vehicle is stopped, measurement under actual driving conditions is difficult. The latter requires characteristic values such as the stiffness and damping coefficients to be identified in advance, which cannot be achieved either easily or precisely. Therefore, this paper proposes a new experiment-based analysis method. This method enables the acquisition of engine torque and transmission torque/force by measuring only the acceleration values of some components under driving conditions. The key to realizing this method is the measurement of motions with the necessary and sufficient degrees of freedom (e.g., pitch motion) since these greatly affect vehicle behavior. Another aspect of this method is the adoption of appropriate equations to express the relationship between action and reaction forces and the measured acceleration (inertial force). Once the torque and force are obtained, a physical model can be devised even if the characteristic parameters of the vehicle are unknown. For the vehicle used in this study, tip-in acceleration behavior can be expressed by a model with only seven degrees of freedom, which expresses the important motions of the powertrain, driveline, and chassis. This model allows a simple understanding of the acceleration behavior and can be used for concept design.