I tried to be clear. I said "mid September before it started to get cold". I have owned this car through 4 winters, so I know what happens to the range due to cold weather. The weather is NOT the issue. Every summer 87km has been the tops indicated on the dash in the morning. It reached that for a while this year as well. In September this year the highest shown suddenly became 65 km, hence the 25% drop. This has nothing to do with cold weather (we had summer temperatures here in September) or using the heater.
You haven't given any information that would lead me to believe that this is battery degradation. In fact I would be 99.99% certain it isn't.
What is your actual range on a full charge?
When you fully deplete the EV range what does the car register as the KWh used?
Then you should look at things like tire pressure. Did you get new tires?
Are the brake calipers sticking?
Possible problem with a bearing or wheel hub causing increased frictional losses?
Did the dealer do a software update that reset some of your settings to default and turned the auto defog on?
Alignment issues?
When it comes to "actual" decreased range there are several things that are likely the cause before battery degradation. GM has never had to replace a battery pack due to battery degradation yet on a Volt. And their battery cell failure rate is one in 2 million. But if a cell had failed your check engine light would be on and it would be throwing a number of codes.