Prius MPG Drop - Master checklist

At around 35k, my 2017 Prius MPG dropped from 60-65mpg to 47mpg. I've been into extreme car modification for decades and I thought this would be a simple fix. I live in Socal and the weather only fluctuates from 65-85 on average. I started with the simple fixes first - reflash the ecu, check the tire pressure. Nothing. Rubbed my chin and took it to a tire shop and got a rotation. Still no change in MPG. Ok.. so I took it up a notch. Replace the air filter with K&N.. nope. Replace the cabin filter.. nada. wth. Nothing installed on the exterior to create wind-drag. I was frustrated at this point and took it up another notch. Replace the 12v battery with a brand new one from the Toyota dealer. This is usually the last step for many and often times it fixes the issue. For me, still nothing. I didn't notice any difference in the frequency of engine kicking in. I believe in 'if you throw enough money at it, you can fix the problem.' So I went and replaced all 4 tires (my treads were at 20%, but had some cracks on the walls). This still didn't fix the issues. My last ditch effort was to clean the fuel injectors. Still didn't change anything. I swallowed my pride and started digging deep into the forums for answers at this point, but still couldn't find anything worth while. So exchanged it for a 2022 Prime XLE, lol. I work from home, so I'll be filling up around 5x times a year. With a $6.50 average gas price in socal, this is awesome. Here's a check list for MPG drop. for 90% of you, one of this will solve your issue. Order from simple to more aggressive.

- Check Tire Pressure
- Rotate tires
- Temp drop or fluctuation?
- Elevation change?
- States with extreme weather - try thinner synthetic oil in the winter and thicker in the summer
- disconnect the battery to have your ecu re learn your driving habit
- Change air filter. Change cabin filter
- Newly installed roof rack or other culprit of wind-drag
- Change all 4 tires
- Change the 12v battery (often times this fixes the issue)
- Clean the fuel injectors (not using injector cleaning solutions, but having the shop clean it)

desperate attempts
- replace transmission oil (dirty oil can interfere with shifting)
- clean or replace O2 and MAF sensors
- change spark plugs.
- clean throttle body

very lastly, it can be your electronics, which isn't common.

 

Also, running AC will hurt MPG by 3-5, but not by 10+.
There's a small possibility that my main battery was wearing out, but it only had 35k miles.