noticeable increase in fuel consumption, losing 50 tacoMiles per tank of tacoJuice... what is going on?

i ought to change the pcv out in my truck
it may be the original because i can not ever recall having it changed ever

not entirely sure if that is causing this severe abnormal fuel consumption, however
If you have any oil leaks from gaskets or seals, it's definitely time for replacement. They should be changed at least every 100k miles. Never heard them affecting fuel economy.
Rear main seal recently was replaced
And I routinely monitor the oil level in the crankcase at every opportunity I am afforded
... happened upon this on a KBB page

1997 Toyota Tacoma XtraCab MPG By Trim​

1997 Tacoma XtraCab Pickup - city 15 MPG - hwy 18 MPG
1997 Tacoma XtraCab SR5 Pickup - city 15 MPG - hwy 18 MPG

this is VERY interesting

the near-fastidious records i have kept since May 1997 prove i was getting much much more than 18MPG on the freeway over a very long period of keeping tabs
Those got to wrong...heck my old v8 tundra use to get 13 city and 17-18 highway. I get as good, or better city mileage in my heavy gen3 v6 tacoma than what you list for highway.
perhaps if i re-install that 80 pound sandbag in the back box i will get back to the consistent miles per tank MPT i have had for so very long
? changes in wind drag would have this notable of an influence on the drop??
perhaps this found here just today might provide us with some relevant clues
more research is needed

searching timmy the toolman videos for possible clues

unless somebody knows about where that hose goes on the buried side and the purpose function of it?
It's probably a crankcase breather hose that feeds back into the intake manifold similar as what the pcv valve does.
that is definitely what it is looking to be
why would it be spitting oil vapours back up that far? it connects to the top of the valve cover in the back
When an engine has that milage as yours, it produces excessive blow-by, a sign its tired, or in need of a new pcv valve....likely both if the valve was never replaced. Even new engines produce some blow-by, it's why all engines have a pcv system. Just consider it normal for what you now have.
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i am certainly going to try and get a new pcv valve in there and see what is what... it is likely the original PCV and i recently noticed that it has oily wetness around the grommet that i cleaned up with wd40
hopefully the grommet was changed 5 years ago with the VCG and not hardened too much from time elapsed so it pull right out with easement