MPG dropped by over 20% on the same run and same variables

You don't say where you're reading the trip MPG, but I assume you are resetting the counter after you refuel so you are starting from scratch.

How far is the trip you are measuring? Short trips can skew the reported MPG, even if you think you are making the trip under "identical" conditions, because the sample size is so small.

The only other thing that makes sense is the recent service you reported. Perhaps the new spark plugs and air flow sensor are contributing to your reduced MPG. Maybe you have a cracked spark plug, broken coil pack or other anomaly that is not firing efficiently, reducing the ICE output causing reduced MPG because the ICE has to work harder. Or, maybe the air flow sensor was "knocked out of whack" by your clearing, whatever that is. A badly adjusted air flow sensor could certainly cause reduced MPG.

The first place I would look is the last thing you did to the car!

 

Load test the new 12 volt

there are a ton of counterfeit plugs out there

 

Others have reported a drop in mpg with new and/or different tires. Would that have this much impact?

 

Did a 12v disconnect or other maintenance directly precede the drop in mpg? I'm working on proving a low mpg mechanism that results from a 12v disconnect resetting parameters in the HV ECU that control EV Output.

Essentially computer settings that tell the Prius to use EV Output from the Hybrid Battery aggressively are reset... due to a glitch or other unknown factor, the Prius has permanently reduced EV Output after the 12v Disconnect and never reverts the software back to using EV Output Aggressively again. This mainly effects City mpg and can reduce it all other factors being constant by 10-15%+.

I made a preliminary video on this issue that I call MAX and WEAK EV States. Video linked below:

I believe this has been an issue for a long time and for certain effects Gen 2 and Gen 3 Prius, and have found recent evidence it effects Gen 4 as well. It seems the more that the Prius ages the more likely the WEAK EV State becomes permanent. I believe there was even a lawsuit based around this issue in 2015ish as described by the LA Times below... but the issue wasn't described properly at the time:

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I had about a 10% loss in gas mileage after running gas from a certain price club gas station. This was just to save like 5% gas cost there. Apparently, that specific chain of price club gas was not top tier gas. After realizing this, I ran fuel line cleaners and switched back to normal branded gas stations with the top tier gas, and got the mileage back after a while. Check the gas quality may be?

 

That'll do it.

What are the new tyres?

 

tyres can cost a few mpg, nut i don't think 20.

 

Run flat tires can cost 20% when compared to LRR tires.

JeffD

 

how is 66mpg to 46mpg 20%?

 

You are assuming only one cause for the drop. The OP original post indicated that it dropped by 20% over a period of time, and included a battery replacement along with other items (including the tires). I wonder if the battery replacement was done by simply switching the batteries (not maintaining power during the transition) without resetting the parameters Toyota recommends (which has, on occasion (not always) resulted in a drop in mileage along with other strange phenomena). Has the ICE air filter been replaced (with a Toyota filter)? Are the plugs OEM or another brand (as you mentioned). Everything the OP mentioned has the potential to improve efficiency or decrease efficiency depending on various factors.
FWIW