I'm not sure what's going on with mileage

Rapid cycling of the battery SOC (state of charge) and decreased MPG's are two signs that can point to a weak hybrid battery. If it's still the original HV battery, plan accordingly regardless.

New, non-LRR tires can cause a dramatic drop in MPG's too. What kind does it have?

Using heat especially over short distances will cause the cold gas engine to run a LOT more which drops MPG's too.

Thicker oil can lower mpg's also.

GOOD LUCK!

 

First thing to check is your 12V battery. A low 12V battery will make the inverter pull extra current from the HV battery, draining it more quickly and costing mpgs. Many posts around here on this.

New tires, especially ones with a lot more rolling resistance will reduce mpg. What tires you have on it versus previously? Also note that new tires usually suffer from worse fuel economy for ~500 miles as they break in. See where your're at after two tanks.

Also, what was involved in the tune-up? A good practice after changing items like the spark plugs is to disconnect and reconnect the battery to reset the ECU to let it remap to changes in the engine.

The Prius engine gets heat starved quickly and will turn on the engine a lot in cold weather with the heat on when stopped. I personally set the heat to 65 degrees with the fan on the lowest setting and bundle up when driving around town in the winter to help keep the engine warm. Even with 75% of my grill blocked the temp will drop fast when it's below freezing out when the car is stopped. On the freeway it's no problem though - water temp reported on my scanguage quickly rises to 85C and stays there - then I turn up the heat. If you haven't yet, look into partially blocking the grill in the winter time.

To me it sounds like a combination of ECU resetting itself, new tire break in, cold weather, and going through a drive through with the heat up that's affecting your fuel economy.

 

Based on your description and the current climatological conditions, I would say that the thermostat is not allowing the engine to get up to operating temperature causing excessive on-off cycling of the engine and the subsequent decrease in FE.

 

it's probably a clogged air filter, no worries, very common in the 2004, especially in oregon. remove it, tap it on the ground a few times, replace, and you're good to go.
all the best!

 

Bisco
I’m impressed, I have seen your posts in a few threads and you seem spot on. Thanks for great advice. I will be using your recommendations for my few issues. (Gas gauge and mileage)

Will let u know outcome. Question. I joined this when I purchased my car in 2015. There used to be mileage threads (100k, 200k, etc). Are any long mileage threads still going? Could not find. My car is 215k+. And would like to hear about other experiences from travelled friends.

Thx. Tai

 

yes, the threads are still here, but search is sometimes difficult. there are all pinned in the gen 2 main forum.
congrats on your 215k, keep her going!