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Im gettin roughly 14-15 mpg in my 99 f350 eclb and 38" rubbers
 
I have a smaller tank in my 95 but I have no reason to drive much with my truck lately so the 20$ will get me just about a quarter tank, I go till its just at the red line and toss another 20$ in it and right back up to that familiar 1/4 mark. If thats not enough then when I go to my place in nh I usually fill up before I leave and hit up an exit just before mine almost all the way there because it's cheaper. I have filled up completely and then refilled my tank when I got there. And I was impressed by how much my truck used, now this was back on the summer so forgive me for not having fresh data but I remember hand calculating and it was roughly 21.7 mpg. All highway around 80ish miles from my house to the gas station and I don't recall it being much more than 15$ and it was around ~3.60/gallon IIRC back this previous summer. Let's just aim high and say 3.70 a gallon to be safe, that brings it to just a hair over 4 gallons. At 80~ miles just just about 20mpg. We were loaded with bikes and gear.
 
The problem with fuel economy threads is people fail to state what exact truck they drive. An F250 2WD single cab manual transmission will get better economy than an F350 4x4 crew cab automatic DRW, obvious right? So just because we all have the 7.3 doesn't make them comparable on economy. Someone saying "Mine gets 18-20mph" doesn't mean anything if we don't know the specs of the vehicle. I'd say a properly modded manual trans light truck could see 20mpg freeway cruising, not average. My 450 might see 13mph max doing a slower freeway cruise, doesn't mean my engine is misbehaving. Just the nature of a heavier duty truck with higher rear diff ratio and the megaheavy 6 mini-semi tires lol.
 
I need some help. I am a mechanic, but not a diesel mechanic. I bought a 99 F250 a year ago and I am not getting the fuel mileage that everyone says I should be getting. In the city, on the highway, loaded or unloaded, towing or not, I have not gotten over 14mpg.I am hearing everyone says that they are getting in the 16-20 range. I do not drive hard at all. I do have a superchip programmer on it but even at the stock setting, or the fuel saving setting, it is no better. Here is what I have done so far:
-Checked every sensor I could find for correct resistance, temperature, etc.
-Checked for exhaust leaks and restrictions up-pipes and all.
-Checked for boost leaks
-Checked the turbo for excess play
-Changed fuel and air filters
-Filled the fuel bowl with seafoam
-Ran several bottles of cleaner through the fuel system
-Checked for dragging brakes
-Checked boost pressures
-Checked fuel pressure -57psi
-Removed and cleaned exhaust back pressure valve
-No smoke in the morning when first started, if there is some it is so faint you can not see it.
- Very little black smoke when you really get on it
-Tires are 32's on stock wheels
The only thing I have not done to date is checked the injectors or injector o-rings.

I don't know what else to do. Help please.
Mud tires, 32s, that will pull most of your mileage down.

I have a 99 manual with highway tires, it's 2WD (what is yours?), and it gets 20.

Each of those heavy tires is robbing you because of wind and rolling resistance as well as the weight.

Then again, if I get stupid with a Lead Foot, I get 14-16. But it's fun.
 
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