

I own a Prius (not a PHEV though, just a hybrid) and can corroborate that my mileage goes down significantly in the winter months. It is a combination of a couple factors in my experience.
- Needing to run the engine more to heat things up for defrosting and heating the cabin.
- Related to the above, I tend to idle a lot more in the winter while cleaning ice/snow off the car, letting it warm up, or clearing the driveway.
- Switching to winter tires (Blizzaks) negatively impacts rolling resistance compared to the LRR tires (Ecopias) I use the rest of the year.
I tend to average ~45 mpg in the summer and ~37 mpg in the winter over the past two years.
Just for some background information on how most countries tend to rely on larger, more rigorous regulatory bodies…
I am in the pharma industry (not in vaccines though). Typically the two main regulators that most other countries look to as a reference are either the FDA or the EMA (the EU organization). This usually means that if you can satisfy the requirements of one of these bodies, then it is satisfactory for the other country as well. However, it isn’t universal as each other country will usually have some modifications here and there for whatever reason. The most annoyingly particular ones I have dealt with in the past are China and Japan.