Putting together a winter emergency kit for your car is a tricky prospect. After all, you have less space than you have for your home kit, and the more weight you take on equals more gas being burned while you drive. Therefore, anything beyond the typical emergency kit needs to be carefully thought out.
Ideally, your car should be equipped to get you through being stranded in the snow for one day. Your primary concerns should be staying warm and hydrated. Food is good to have, but less of a priority, since you should be able to go for a while without eating. You can’t go as long without drinking water. Further, though it may seem feasible to rely on the snow around you for emergency water, this is going to further sap your precious body heat in situations where your heating system has failed. Therefore, try having a bottle of water and a thick, winter blanket handy.
You can also benefit from having a few other emergency supplies, like sand, an ice brush, an ice scraper, and snow chains.