How much is a reasonable drop in mpg when old man winter sets in on my cars?
Just an opinion,
I didn’t think the ambient temperature affected the mpg enough to be measured. The engines on newer cars warm up fairly soon and the air density doesn’t change that much over the typical temp range we see. If the mpg was seriously affected by the changing of seasons the government mpg ratings would have to account for this on the window sticker.
MPG is very much affected by outside temperature. If it’s 20 degrees outside your car spends much longer in open loop running rich than if it were 90 degrees outside. So temperature really makes a big difference if you make lots of short trips. Starting in 2008 the EPA started figuring ambient temperature into their mpg estimates.
There are a number of factors. One is the fuel. Winter blend fuels (diesel and gasoline) generally have lower energy, so you get lower mileage. That varies greatly depending on where you live. Local temperatures and more important, the length of your drives will also be a big factor. We all tend to have shorter drives during the winter.
I would suggest that anything over 10% is large and less than 3% is small. These small numbers are difficult to measure accurately.
I didn’t think the ambient temperature affected the mpg enough to be measured.
I take it you live in the South.
Cold weather can have a drastic effect on mpg. I usually see a 2-3 mpg drop. If it’s snowing a lot…3-5 mpg drop.
I usually see a 10-15% drop in mpg, but my car is a little over 20 years old and rather well worn. Newer cars may not be as profoundly affected, but you will see a difference if you normally keep track of things like this. I track mileage with every fillup, and when the weather is bitterly cold I may only get 270 miles out of a tank as opposed to the usual 300 in the summer.
Cold, dense air increases wind resistance.
Typically in my 4-banger cars my mileage will drop 2-3 mpg in the winter, but it’ll vary widely depending on the vehicle, the type of driving you do, and the driving environment. My driving is mixed, but if you live in North Dakota and your driving is all local you may rarely get to full operating temp. I remember when I lived there we used to put cardboard in front of the radiators just to get the engine to warm up enough to get some warmth in the car.
less energy per volume, to be precise (and anal) 
Ooops, I stand corrected! (and yes, most of my driving has been south of the Mason-Dixon line)
Given that absolute zero is ~ ?460?F
and the ideal gas law PV=nRT,
then it follows that 92?F air is 20% less dense than 0?F air,
and that’s pretty significant.
The mileage in my Lincoln has actually gone up about .7 to 1 MPG since cold weather hit.
It’s averaging about 25.2 combined with an 80/20 mix of highway/city whereas before it running about 24.2 to 24.5