Even though the OP posted in Jul, I did not see a conclusion, so I'll add some two cent comments. OP as you noticed from the above responses, it could be the tires causing the mileage drop. Excess ICE idling (or high speed test drives) could also skew you MPG computer calculation. From my memory, I believe there was a recent (year 2020) recall (that some reported did decrease MPG) for the invertor (to prevent engine stalling in traffic) that was a software update. You could: 1) Check the treadwear rating on the old versus new tire. 200 rating is "sticker" than 400 and would reduce mileage IMO. High rolling resistance is one of the key tire words for Hybrid's stellar MPG. 2) As noted, you could increase tire pressure in increments, noting your MPG and center of thread wear. Good luck.
If you go to the Toyota site you can put your vin # in and see what updates you have or need. They are suppose to check everytime a car comes in.
I ran chinese tires and lost about 7 to 10 percent mpg... do paying for better tires saves you over the lifetime of better tires in gas.
This is water long past the bridge, but: The speedometer reads high on purpose, due to legislation in place to ensure you’re never being told you’re going slower than actual speed. The odometer is reasonably accurate, again due to legislation. My speculation: the car’s internal knowledge of mpg is reasonably accurate, but Toyota intentionally exaggerates it.
I think a year is long enough that the farmers downstream have dumped the water on their fields, the corn has transpired it back to the atmosphere, the trade winds have blown it over mountains and plains, it has precipitated back as snow last February and has finally found its way back to the bridge. Unless the bridge is upstream from Lake Oroville.
