i drive easy, and with stock engine get about 13 city and 15 highway.
i also do my city driving with OD off and keep rpm's up around 1800-2000 at 45-55 mph. with OD on it bogs down at 1000-1200-1400 rpm

did you ever get decent mileage, or has it always sucked ?
is your front 02 sensor connected and working ? it controls fuel mix....

i don't think there is any noticable difference in mpg between 180* and 195*.
tire pressure of 30 psi is adequate, and anything higher than that is insignificant in city driving.

 

Dude, CHECK YOUR PLENIUM GASKET. Mine was burning oil slightly, and my cat and oxygen sensors were shot. I bought a pleneum gasket repair kit from Hughes Engine. It comes with an aluminum plate to replace the steel one. Since then, my mileage is around 19MPG highway.

 

Yup my truck didn't seem to like the Autolites either. I went back to Champions and like them much better.

 

Tire Pressure info is on a sticker on the door jamb. driver's side i think

 

I still have a few issues to resolve (cat, muffler, air filter, tonneau, etc) to improve mpg, but last I checked i got 518 on 30 gallons. that's roughly a little over 17 mpg hwy. put 1/4 tank in with some techron plus and i'm at 11 miles so far and the needle hasn't moved, with the exception of going up or down a hill.

BUT i did the plenum fix, cold air intake mod, new timing chain, royal purple, champion plugs, summit brass cap and rotor, 8 mm wires and probably some other things that the clear coat fumes i sprayed earlier are knocking out of my brain lol.

I also have a vacuum leak or two that could very well be affecting performance/mpg.

I like my truck.

 

I have a K&N drop-in filter, 180 degree t-stat, Hypertech 87 octane program, MSD wires, NGK plugs, Fastman Throttle body. I drive about 55 miles rt daily to work and back about 70% highway at 70mph. I run my tires at 42 psi, use 87 gas, and don't baby the truck. I have been averaging 12.5 - 13.1. Truck has 103k on it now. I was getting 11mpg when the plenum gasket was blown and the truck ran like crap. Your tire pressure definitely hurt you - but not 3 mpg worth. I'd check the plenum gasket...

 

See, I keep hearing about this plenum gasket thing but I don't know anything about it. I haven't searched the board about it either. I don't hear any weird noises, see any fluid leaks, or anything like that so I assume it is okay. Am I wrong in assuming that without further investigation?

 

I'm not sure what the gas mileage was like before the modifications. I'm kicking myself now for not checking before. I'm running 42 psi in the rear tires and 45 in the front. I'm switching back to the 195* t-stat this evening and then I'm going to top it off. Hopefully that will help but if it doesn't, the plenum gasket might be the problem I guess. I'll have to research it.

 

Well do a search on the plenum gasket thing! Because that is most likely the problem. There is info all over this website about it. There is more than one way to check it, but the easiest is to remove the air cleaner and open the butterflies on the TB and look down inside and see if you have any oil pooling in the intake.It is not a leak youi will see on the exterior. Plus it's a 98, so it's almost guaranteed to be bad.

 

Easiest thing to check. Basically, I wasn't positive because I didn't have more than maybe a mm or so, heightwise, of oil in there. But since I replaced it with the aluminam pan, it's shiney down there. A tad bit of discoloration near the back or center isn't so much a big deal. But definitely search it, there is a TON of info about it around here.