Car sharing and ride sharing will likely increase the annual vehicle driving distance, which in turn may accelerate passenger car retirement. In this study, we develop a semi-empirical lifetime-driving intensity model. We integrate our model with a carbon footprint estimation model, which assesses future decarbonization of global, European, and Swedish energy systems. We calibrate the lifetime-driving intensity model using Swedish vehicle retirement statistics, which suggest that driving intensity strongly impacts vehicle lifetimes. Our combined model shows that use-phase emissions depend on driving intensity, while manufacturing emissions are distributed over the cumulative driving distance. Hence, the carbon footprint depends on both the calendar age and cumulative driving distance of the vehicle. The results of our study also show that higher driving intensities generally result in lower carbon footprints. However, empty travel by shared autonomous vehicles, i.e., the additional distance traveled to pick up passengers, may cause carbon footprints to increase.