'90 dodge w250 highway economy,help!!

O2 sensor is not going to be silver bullet you think here. BTW, I have a 89 4x4 burb with over 180K mile (that I bought new) and I have NEVER changed O2 sensor on it and it is getting some of the best MPG of its life now and will do 18 MPG on road all day long. Might change it for grins one day but it does run great now and I am afraid to mess with it.  

never said it was the silver bullet. don't care about your suburban on a dodge forum. does no good to compare a suburban to a w250. you sound like every one that comes into my dads shop. don't wanna fix some thing that you think is not broke. its called preventative maintence  

1978k-20:
never said it was the silver bullet. don't care about your suburban on a dodge forum. does no good to compare a suburban to a w250. you sound like every one that comes into my dads shop. don't wanna fix some thing that you think is not broke. its called preventative maintence

No you do, the point is that O2 sensor is not a silver bullet and Dodges of that era are not known for good MPG and do not get his hopes up.  

your the one trying to get his hopes up with your gss mile age of the suburban. never said the o2s were gonna be the silver bullet you did  

No I was not as I said Dodge of the era are know to be thirsty. It was because of low compression and retarded spark timing to tolerate 87 octane fuel which lead to lower MPG. GM used more aggressive timing because they had active knock control to limit consumer complaints on low octane fuel, dodge did not so timing was retard all the time to prevent it from knocking which again hurts MPG. Dodge was kinda slow figuring this out and a lot of their vehicles suffered from it for many years.  

well it's been a year and little got done. put a flatbed on I bought out of a farmers weed pile. put a thrush glass-pack n a down turn right where the catalytic was n cut the trash off. put a water-pump n a radiator on. took the air pump off. n the blessed thing over-heats in 5 miles. that happened at the confluence of cold weather n no more money so haven't worried about it tho I sure could use it for wood gathering. oh well, get some money n some rack time n see what's wrong.  

Glass pack did not improve MPG any in town/urban setting and may have hurt it more. Gas engine actually like a little back pressure for best lower and middle RPM response and MPG. A engine is driven by capturing energy from expanding gasses as piston travels down on power stroke. Let gas pressure bleed off too quickly before piston reaches BDC and efficiency drops and lower RPM torque does too.  

I agree with sno you gotta have back pressure in a gas engine.The packs arent helping you.You might want to make some pipes that come out the side to gather a little presser back.Dodge truck mpg sucks in 80s and 90s trucks so it might be just that truck.I've seen a 87 2 wheel drive one with a 318 and stick get 15 at best.  
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Put the air-pump back on-it needs the fresh air as it has air holes in the head, under the exhaust manifolds!! Trust me..I learned this the hard way!  Also, make sure the intake heater flap on the passenger side exhaust manifold is not stuck in the closed position.  It is located right where the pipe and exhaust manifold meet.  You should be able to move it by hand.  If not, spray some WD-40 on it and carefully tap if with a dead blow hammer to get it moving again.  Did you replace the thermostat?  If not replace it.  Also, did you put the correct mix of anti-freeze/water if the radiator?  Did you fill the new radiator up, then start the engine and bring it up to operating temp, then re-check radiator fluid?  Do this careully after you shut it down, as the fluid will be hot, and there will (should) be pressure so just crack the radiator cap until the pressure bleeds off, then open it up to check it.  Do you have shift on the fly 4WD?  I so, you have a vacuum operated front disconnect axle (Dana 44 F8J)  You can increase your mileage by installing as set of Warn locking hubs.  Also, make sure ALL of the vacuum lines/air hoses are not leaking.  Unfortunately, these motors are basically smog pumps that are designed to move a vehicle.  One air leak and it all goes to hell.  One more thing to check is your EGR valve.  It should move by hand.  If not or you hear air coming thru it when you squeeze it, it's shot so replace it.

1990 W250 Club Cab
360 ci
2 inch Dynamax headers, plumbed for air system and o2 sensor.
4 inch Tuff Country lift
31 x 10.5 Dean Cooper m/s diggers on 16in Modern rims
126k miles and gets 14 mpg average between hwy/city driving.