In this paper, a simple vehicle model was investigated for the study of vehicle behavior in the straight-ahead motion. The vehicle model has two degrees of freedom without any suspension effect included. Through the analyses of the model with the various tire properties, we can explain how force and moment characteristics of the tires can cause lateral drift in the straight- ahead motion in free control mode. In the second part of the paper, a commercially available car was selected for the validity of the analyses. This car was modeled in detail with one of the multi-body dynamics simulation programs. The simulation results show that the amount of vehicle deviation from the intended straight path is directly related to a characteristic of the front axle tires, known as ATSP (Aligning Torque Static Phase). In this particular simulation, linear regression analyses between the ATSP of the tires and lateral drifts of the simulation with three different forward speeds give the coefficients of determination higher than 0.95 in all cases. For the purpose of vehicle test, the same twelve pairs of tires were mounted to a vehicle. Vehicle tests under the same three different forward speeds were done on a flat asphalt road. From the test results, the relations between lateral drift and ATSP of tires were examined. Linear regression models have the coefficients of determination higher than 0.78 in all three cases. Even though there is difference in the slope between simulation results and test results, it clearly shows that ATSP of the front axle directly influences the lateral drift of the test vehicle.