Horrible Gas Mileage on 1991 Toyota Pickup v6 3vze
 

I am getting 8 MPG on my 1991 Toyota Pickup V6 3vze Fuel Injected Engine Automatic Transmission 4 x 4. Truck has 199,892 miles on it. I had the truck tuned up by my mechanic in 2019. They replaced spark plugs, wires, distributor cap, rotor, and checked timing. After they tuned up the truck my gas mileage went from 10 mpg to 8 mpg. I took it back to them and told them my gas mileage went down after the tune up. They still did not fix it. I also replaced the O2 Sensor on it at that time.
My problem is after reading all the posts here a lot of people have the same problem but I do not see any responses as to what worked to solve it.

My nephew has worked on the truck and found the mechanics did an awful job on the tune up. He did a new tune up on it.He also found the fuel line to the fuel pressure regulator was kinked. We replaced the fuel pressure regulator and the fuel line hose. Funny the mechanic never spotted this. Result was a better running truck. Thought it would have helped with the Gas Mileage but still getting 8 MPG. Nephew checked the timing on it and it is at factory spec Compression test results 150 on all cylinders. So we are stumped as to why the truck is still getting horrible gas mileage. He checked for error codes and pulled a 12 and 24 error code. Another issue is that when the temp outside is hot-since living in Arizona. the truck runs sluggish versus when the temp is cooler outside it runs great.

Any suggestions would be helpful.
Thank You

One important thing is to determine if the ECU is putting fuel management into closed loop when the engine is warmed up.
Does your engine have a proper thermostat installed?
Valve lash and compression numbers should be determined as well.
The 3vz engine is able to achieve decent fuel economy when it is operating as designed.
My '95 3vz 4Runner with 4.56 gears and manual transmission regularly clocks 19-19.5 miles per gallon on 10% ethanol fuel.

Thermostat has not been changed in awhile. Compression test result 150 on all cylinders.

When it comes to mpg there's not a ton to check (assuming the vehicle isn't throwing codes.)
1) timing
2)sparkplugs
3)tire pressure(just so you can say you did)
4) off brand / cheap gas(though the truck engines were pretty forgiving on this)
If those all look ok I'd start looking at the possibility of inferior tune up parts, especially plugs. If it has anything other than ND or NGK I'd toss. Bosch were notoriously bad performers in
Toyotas bitd