Getting 32-38 MPG

I have a 2013 Prius 2 model. I've owned it about 2.5 years now and Its MPG seems to be going down for some strange reason. I have been running tests to see what could be the contributing factor but seem to be stuck as all have been negligent in change. The car has 97k miles and I test the Hybrid battery health every few months. Each time tests come back great. No issues, battery at 85% health.

A few factors in consideration below.

• PO lift installed at right after purchase - lift did NOT affect MPG
• Larger 205/70/15 All Season tires installed. Always kept just below Max PSI - Did affect MPG, decrease of about 7 on both highway/city
• Roof rails & loaded basket installed - reduced average MPG by another 7-10

I have tried the following to help increase MPG:

• Removing roof basket/unloading it - little change
• Removing roof rails & basket completely - increased by 3-5
• Driving in Eco mode, and at or below speed limit/hypermiling - I have always done this
• Intake/cabin filters brand new
• Did EGR/Intake & Cooler cleaning at 90k miles - no change
• Weather - car is kept in climate controlled garage. Cold winters are a bit rough, but change is quite negligible unless trips are short. Same with summer.

I need the roof rack/basket for long haul bi-montly trips. I'd absolutely love to remove it, but I need extra cargo space. Seeing as this only seems to reduce my highway MPG slightly, I'm not sure if it's a major contributing factor as explained below.

Is there any other factors I could be looking into? Even after multiple tests, with this car completely stock (aside from slightly larger tires) It only gets at best 38-40 MPG city and 33-35 MPG at 70 MPH, and obviously worse with the rack attached.

Appreciate any insight or recommendations!

 

I'm planning on installing a 2" receiver hitch later this summer and will definitely try this! My only concern is garage parking. Depending on how far it sticks out while folded, I might have to remove until I have to use it for trips. a little annoying, but would be much easier than removing the rack, that's for sure.

 

PO lift is the Prius offroad lift. I installed it and new larger tires at the same time, roughly 1k miles after purchase. I did not take in account for size comparison revolutions, so that makes sense! Cargo is lightweight (<50lbs) but require bins that are larger in size.

I guess my concern was even after removing everything for a 1500 mile test, I was still receiving pretty low MPGs. I guess the tires alone are the major contributor.