Summary: We present a kinetodynamic quarter-car model for passenger cars with McPherson suspension, which includes geometric nonlinearity due to the constraints imposed by the interconnection of elements. We retain flexibility in the strut as well as the tire, and use the simplest linear constitutive relationship for these flexible members. Accordingly, our tire is modeled using only two springs with linear stiffnesses; one is aligned along the road surface while the other is perpendicular to it. The strut is modeled using a linear traditional Kelvin-Voigt element, while other suspension elements are assumed to be rigid. The equations of motion for the resulting two-degrees-of-freedom system are derived using the Lagrangian formulation, including the potential energy associated with gravity. The gravitational forces result not only in constant external force but also appear as parameters in the coefficient of various terms involving the generalized coordinates. We validate our analytical model by comparing the vibration characteristics of the linearized model as well as the time-domain results of the fully nonlinear model against comparable models in the multibody dynamics software MSC Adams/View. We study the importance of geometric nonlinearity in capturing the true response of the vehicle body by comparing results from the fully nonlinear model against those from the linearized model both for harmonic as well as standard road inputs. Finally, we highlight the importance of including gravity in our formulation by pointing out the inadequacy of the models without gravity in capturing the true vibration characteristics (both linear and nonlinear).