Poor Gas Mileage

I am working on a relatives 2005 Prius, and they are experiencing a dilemma where their Prius always has had poor gas mileage (as of right now, in the winter, gets roughly ~28-34 MPGs, summer ~38-40 MPG). The car itself just hit 190K so I volunteered and did a spark plug change, changed the oil (it was time for one anyway), changed the PCV valve (it was really clogged up). After doing this, my friend noted the car definitely feels a lot better, but the gas mileage hasn't improved significantly. Several things they mentioned was they had swapped the hybrid battery at a local shop with a refurbished one, but the warranty just expired. From my general testing, the hybrid battery definitely holds the charge, and when driving around there is no evidence of a bad hybrid battery (I have been in Prius's where the hybrid battery is going bad or is pretty much done, and no evidence of this was visible).

Several things I did notice though (and this may be the large reason for poor mileage) is that my friend only uses the vehicle for ~ 3-mile commutes to work, thus only putting ~6-10 miles on the car on a daily basis. I figured due to the short commutes, that can be the root cause, but hitting ~28-34 MPGs right now doesn't seem to make sense to me. The tires can be replaced, but I'd say there are at least ~5,000-7,000 miles on them based on the tread, give or take. I also hooked up Techstream and not a single code was found.

I am basically running out of ideas, and my friend was wondering if they should consider selling the car for something else, and I personally am growing curiosity as to what can be causing unusual MPG's for a vehicle with a hybrid battery that was just replaced, and an engine that doesn't show any evidence of strong amounts of wear and tear.

 

But those numbers seem unreasonably low. I used to commute daily in my old 2007 (which has long been sold) and I had a stint where I only used the car roughly ~15 miles a day give or take (usually just a 4 mile commute one way to school add on running some errands and what not), and the difference in MPG's was going from ~48-49 to ~44-45. Anything <35 seems pretty low, no?

 

B U M P

 

Not entirely sure if the issue has always existed, but I recall them bringing this up for the past year. And the car was owned since new, but it was handed down from his parents. But I acknowledge that there is a multitude of variables that go into gas mileage, but figured maybe there is a common thing I can inspect/fix (in this case, I thought maybe a PCV Valve replacing can do it). But even then, I have seen in person 2004s and 2005 Prius's kicking it with 250,000+ miles hitting 45+ MPGs, granted those are being used in different ways then this vehicle, I just don't think age and current mileage would be the sole diagnosis, especially since the hybrid battery and engine both seem to be working perfectly fine.

 

Appreciate the sarcasm.