Considering ditching my 2018 after owning a 2007 for 16 years

Man. I hear you. I'm reluctantly selling one of our Gen 2's but 4 cars with 2 drivers is too many. If I HAD to buy again I'd buy a 2020 Prime Limited. Currently looking at a M3. The Gen 2 is rock solid and we are keeping one of ours...for all the reasons you just mentioned. Good luck to you.

 

Could be as simple as different tires

 

different from the 2007

 

The cars driving history data plays a big part in what kind of mpg can be expected from a Prius.
One thing you can check are the odometer trip meters to see if any of them show different mpg ratings.
Typically, the slower a Prius is driven the better the mpg readings will be. But if a Prius has been driven faster for several years the car kinda settles into predictable mpg area and to change that prediction the car is making from the way it's been driven for several years takes a long time before a new driver can see mpg improvements.
My sig other drives the Prius like a regular car. So to keep the cars mpgs in a range that I can live with I have to drive the Prius a lot slower than I normally would whenever I can.

 

mpg's measured at the pump shouldn't reflect anything except driving parameters for that tank

 

There hasn't been to much public discussion about Prius History Data, where it's kept, what data is stored, and for how long data is kept. Some discussion, but not anything the average owner normally remembers. unless they have the diagnostic software the dealers use, techstream. It's true that some setting and data is lost when the 12volt battery is disconnected, but there is other data that is stored for the life of the car and can't even be cleared with techstream. And history data is one of those mysterious things about the car that is probably only well known by toyota engineers that programmed how the systems work in unison.

 

that data would be in the proprietary black box.

 

A little late to the discussion but how many miles on the ‘18? A couple of things I’ve noticed with my ‘22 is it has a lot of little bells and whistles that can be turned off that affect the mileage by reducing the 12 volt accessory load (not a huge amount but some), and when we drive 70 as compared to 55 there’s a 20 to 25 % drop in mpgs!
Additionally, even though my wife and I have similar driving habits we experience approximately a 10% difference between us. Don’t know if any of this helps.

 

My wife has a 2017 LE and I have a 2021 AWD and mine gets significantly better MPG....usually over 10 mpg more. (If I pay attention to my driving, I can actually get over 70 mpg on my 40-mile commute to work which is over mostly rural paved roads.)
Her 2017 has had the engine oil changed every 3,000-5,000 miles (sometimes less than 2,000, I change it every 6-months or 5,000 miles), I drained and filled her radiator coolant and transfer case coolant at 5-years as well as the transmission. I've cleaned out her MAF Sensor and throttle body and checked her brake pads to ensure they aren't rubbing, which decreases MPG.
It's a mystery why her car doesn't get much higher than 50 mpg but, still, that's higher than any other hybrid car I'm aware of. (A co-worker has a Ford Fusion and he told me he barely hits 35 mpg.)
Maybe tires does have more of an impact than I'm aware...I did replace her OEM Toyo tires with General Altimax a few years ago. (And they seem to be holding their tread much better than those OEMs which were almost bald after just 24,000 miles.)
There's a video on YouTube by The Car Care Nut and he teaches how to use the "pulse and glide" method of driving with hybrids which is what I do to get my 70+ mpg when I'm wanting good mileage. (But I usually drive like a bat out of heck so she's only getting around 60 then.)

 

If I can keep my speed below 60 mph, I get close to 70 mpg in my 2021 Prius Prime Limited. I can even get 75 mpg or higher. I get 120 mpg in freeway stop-and-go driving in the summer.

How fast are you driving? Don't expect to get above 50 mpg if you drive 75 mph. Gen 2 wouldn't even get over 40 mpg at 75 mph.

Other than that, if the engine is not properly broken in, its lifetime fuel-economy would be reduced.

Moreover, the spark plugs need to be replaced at 100,000 miles. That could be the main reason for your inferior mpg. You may need a fuel-injector cleaning as well.