Without addressing the time of day or amount of data, it sounds like the car is acting like any device on the network. We have 960 down/40 up and any device that decides to upload a lot of data will eat the entire 40 Mbps upload. It happens when one of my kids uploads a 1GB video to YouTube and their iPhones even do it when they are syncing photos/videos to the cloud. Unless your upload bandwidth exceeds the bandwidth of the destination or its connection or your WiFi speed is too slow, any upload will tend to eat up the entire upload bandwidth on moderate upload speeds (like less than 100 Mbps). As mentioned, the solution (other than being able to stop it or pick an off time) is at the router level. You need a router that offers QOS or bandwidth limitation by MAC address. The router has to be told not to saturate the upload bandwidth otherwise any and every device that is fast enough (and has a big enough upload) will do the same thing to your connection. Those devices are just doing what they are told: send data. The router is what is giving them full bandwidth.
Mike
Mike