sudden decrease in mileage

Like Evan suggests, it's hard to help without knowing all the details.

As the outside air cools, so does the air inside the tires. Cool air is denser than warm air, so the resting tire pressure, first thing in the morning without driving, will also be lower. How much depends on what it was the last time it was checked. If you were running around all day and happened to check the tire pressure in the afternoon, when the tires were hot, and set it to 40 psi, say, at 30°F the next morning, the tire pressure might be down to 28 psi (this is an example only).

When it's cool/cold outside, the ICE runs more to keep it and you warm. It wants to keep emissions low, you want to be comfortable. Gasoline is burned in any case.

To keep air quality better, most states require alcohol to be blended into winter gas. The exact amount may vary, but alcohol doesn't have the energy value of gasoline, so it takes more of the blend to go a given distance.

It's really doubtful anything bad has happened to your Prius, other than winter.

 

I found that in my case where I really lose mileage is during the warmup cycle. The car takes longer to warm up, so those 15 minute trips that use to get 50 mpg now get 40. On longer trips the mileage drop is negligible.

Again, more info on what kind of drop and what kind of trips you take would be helpful

 

It is really cold here also. My mileage has also dropped a lot. 42 or 43 range. That is the lowest it has ever been. I did put air in the tires today. Hope this real cold air moves on soon. Even 30 or 40 degrees sounds good now.

 

Tire pressure should be checked monthly in any case.

 

Low temperatures alone will cause a significant drop-off in mpg, due mainly to the longer warm-up cycle but also due to heater use and the fact that the engine must work harder to maintain its temperature. Loss of mpg due to colder tires is likely to be negligible, but check 'em anyway, as Richard said.

Also, winter-formula gasoline contains less energy than summer-formula (in Colorado, winter gasoline is 10 percent ethanol, meaning about 3 percent less total energy). That will also cause a decrease in mpg.

 

That doesn't sound too out of line. Over two winters, the drop on my car has been a little over 7 mpg, and when I did the math I was surprised it was that little.