A methodology for obtaining the LTE-V2V Mode 3 semi-persistent scheduling (SPS) period that fulfills the string stability condition in cooperative adaptive cruise control (CACC) is presented and validated for platoons with different vehicle types. Results show that the required SPS period depends on: the control loop parameters, the platoon kinematics, the relative ordering of the vehicles within the platoon and the communication parameters such as the modulation and coding scheme, the packet sizes and the number of assigned radio blocks.