I am planning to travel by a rented car from Milan to Reckingen-Gluringen, Switzerland on 8 December. What should I expect, and is it a good idea?
If you really want to go 'only' by car, you will have to approach from the south. Coming from the north, you would either have to cross the Grimsel Pass on route 6 or the Furka Pass on route 19. Both are already permanently closed for the winter. As an alternative to crossing the Furka Pass, you can however use the train shuttle service between Realp and Oberwald, bring your car onto the train and go through the train tunnel instead of over the mountain road pass. You will still reach an altitude of 1550m in Realp. Chances are high, that you will meet snow there. The weather forecast for the coming days predicts snow and temperatures between -5°C and -10°C in Realp.
Coming from the south (Brig), the road between Brig and Reckingen-Gluringen should be pretty straight forward. You will reach your highest altitude of about 1350m in Reckingen-Gluringen. To get from Milan to Brig, the only reasonable road is however via Domodossola and over the Simplon Pass, wich will bring you over 2000m. The pass is in theory open all year, but may be closed on short notice if weather is bad. You must expect snow lying there and depending on the current weather (which we can't predict), the road may be ploughed and dry, or covered in snow. Also here, there is a train shuttle service from Iselle to Brig with which you can bring your car without actually going over the mountain pass. Departures (every 90 minutes) are less frequent than with the other service between Realp and Oberwald (every 30 minutes).
If it is a good idea is a matter of opinion. I would not hesitate doing the trip and perhaps opt for the northern route with the train shuttle. If you however have no experience driving in snowy conditions, and I expect you don't, since you otherwise wouldn't have asked the question, practicing on steep, slippery serpentines in the Swiss mountains is perhaps not the best way to start?