00(1st Gen Taco) Horrible Milage! I'm stumped

Update: Now throwing the notorious PO300 Random Misfite Code(Although it is is still running/idling well with poor milage)

After exploring various forums & from my own experince I'm pretty much at a loss for what the issue could be at this point would appreciate any input fellas, Thanks in Advance!
Situation is I was battling a misfire for quiet a while & now finally having solved it I am still getting only 12-13 MPG, which it used to get 15-16city & 18-20 HWY. The engine starts up quickly, idles fine & runs smooth.(No shaking, no sputtering, responds well on the throttle. Also I very rarely push it past 2500 RPM as gas is so pricey atm.)
Here's a list of what all I've done recently & yes I've ran new gas through it twice since having changed out some bad parts.
New- All 6 Injectors
All 6 Duel ground Spark Plugs(gapped.42- .44)
One ignition coil which the others have been tested(resistance read fine)
All 3 wires for spark plugs
Fuel filter
Pcv Valve
Bilstein Factory size shocks(Yellow&Blue)
Standard Muffler replaced
Valve Cover & Spark Plug Gaskets(Cleaned aswell)
Starter, Alternator & Battery
Timing Belt Kit(Done by trusted mechanic within past year span)
Thermostat replaced within past 2 year
Throttle body, MAF Sensor, Air Filter & Upper/Lower Intake Manifold Cleaned up
Radiator has been replaced(Fluids topped off & no bubbling when cap is off)
Fresh Oil Change with Filter(GTX High Milage 10w-30 is what I've always used)
Fan Clutch Tested Perfectly
Even ran Seafoam through the Intake & in the gas tank.
Again it is running smooth & fine with no Codes being thrown so pretty much stumped at this point.
Side note - Tires are slightly bigger I've always ran the Nexen Roadian MTs 31x10.50R15s & was getting the good milage stated above^.

 

Have you tested your fan clutch?

 

I have not since it gets up to Temperature in a timely manner & always stays slightly below the middle of the gauge.

 

As I understand it, it could still be suspect if it's failed and not operating to spec. I would test it to make sure it's good.

https://www.youtube.com/watch?v=WzhlACxqeqQ

 

Although I am unable to do the Cool-Down Stop Test atm since I've driven it recently today, It has passed all the other Test flawlessly leaving no suspect of it going bad.

 

Not sure where you live but the only thing that changed in my case is the fuel.

I now get terrible fuel mileage in the single digits . The more the price increased the worse the mileage became.

Is it possible someone is borrowing some of your fuel to be returned at a much later date??

 

Good thought which I thought may be the case though the milage has been consistent between 3 different Gas Stations which were Casey's, Jiffy Trip & even our new VP Racing station.

 

You missed my point !! My mileage does not change no matter where I fuel enough to get excited about.

In theory all the fuel in a region gets the same blend .

 

Oh lol Fair enough, I haven't heard of any freinds with less milage nor any reports of such in our Community page on Facebook. Im in upper North/Eastern Oklahoma.

 

Pro tip:
If you hit “quote” in the bottom right hand corner, the person you are replying to will get a notification.

 

You really need to check your fuel trims and other pids on a scan tool.

 

Exhaust leak?

 

There is a pea sized hole on the welds from the pipe to the Muffler. I'm letting some Exhaust Sealent cure on it now. Being that it's behind both O2 Sensors I didn't think it would effect it but will update once It's set & I can drive it for a while.

 

I noticed you haven't mentioned anything about your driveline components. Have you serviced the transmission and or clutch? Replaced the gear oil in the xfercase & diffs? Inspected the bearings throughout the drive line? How are your brakes? Could be a sticky caliper or wheel cylinder, etc...

 

Good question thanks, It's an Automatic I actually blew it up after years of abuse & being at 286k so its been replaced recently with a salvaged one at 100k, was checked out by my mechanic & has no problems shifting. Carrier Bearing holding the drive shaft has been replaced within past year or so along with all the U-Joints at this point I believe, which there seems to be no play in it. Rear axle was also replaced since it was cheaper than fixing the rack n pinion gears in it so it got new fluids about a year ago or so though I haven't checked levels on that since then. As for brake pads & rotors I replaced them within the past year or so, which there is no grooves in the rotors & pads look like they still have good life & even wearing.(Brake Fluid is topped off & responds well)
Side note- Speaking of the transmission if anyone knows places that buys old blown Trans lmk, haven't found anything in my area in North-Eastern Oklahoma.

 

What are you running for tires and tire pressures? Any changes?

 

Same as I always have the Nexen Roadian MTs 31x10.50R15s & was getting good milage with them before. I usually keep them at 35psi which is recommended haven't checked in a while tho none look low. Will check pressure once the exhaust sealent cures.

 

I can almost say for sure that its oxygen sensors. If you replaced them you may have the wrong one.

 

Those I have not touched, it runs really well & isn't throwing a code atm so haven't thought to replace them. Given those bolts are rusty as hell being on the exhauat I haven't bothered to spruce them up either, worth a shot to try to take em off & clean them?