Let's stop some myths.

1. There are no metal filings in a modern quality automobile!!
2. Tier 1 gas is also a lot of BS, sorry. All gasoline sold must meet a standard. In a 20,000 gallon tank of gas the additive by company (e.g., the brand) is only FIVE GALLONS. I worked at a refinery.
3. There is some difference in NE winter blends of gas versus Summer. But in now way will it change things 8-10 mpg.
4. If its cold you get no EV benefit. An engine block heater would probably pay for itself in this circumstance. Terrain also matters, number of stops when coolant is to cold to allow EV mode.

BTW my 1200 Datsun Coupe, 1971, POE $1795 (no heater) and 1.152 litre (rounded up to 1200cc), 1600 lb death trap never got more than 31mpg; usually was 28-29. Things have improved quite a bit. The NAH is 3500 lbs.