hEY EVERYONE! i HAVE AQUESTION ABOUT MILAGE.. i HAVE A 1991 7.3 WITH AN 11 FOOT FLATBED... i AM GETTING 10MPG (MOSTLY HIGHWAY).... THAT CAN'T BE WHAT IS SUPPOSED TO BE??!! ANY IDEAS OF WHAT i CAN DO TO IMPROVE THIS AND WHAT IS IT SUPPOSED TO BE???....

 

A couple of questions first.

What gear ratio?
What transmission?
What kind of terain?
How fast do you drive?
What are you hauling normally?
Rebuilt IP or injectors how long ago?
How many miles on the engine?
2 wheel drive or 4x4?

With an 11 foot bed, I can guess it must be a cab and chassis with a dually rear axle.

 

Finally got my 91 F350 with the 7.3l N/A on a run up the valley last week. Only getting 15 mpg highway with my E4OD tranny and 4:10 rear end. I was a bid disappointed and was hoping for more.

10 mpg sounds a little low to me. Some more information on your truck would help shed some light on the situation.

What RPM is your motor turning at 55 mph?

 

my 90 4x4 f250 sc zf5 & 3.55s gets 18 around town as a daily driver.

 

I averaged 10 MPG with mine 6.9 turbo, T 19 4 speed, 3.55's, and 33" rubber.
New injectors and rebuilt IP and it is up to 13 MPG at the same speeds.

 

NOT SURE WHAT KINDA GEARS i HAVE OUT BACK, BUT GOING 70 MPH (WHICH TAKES A WHILE TO GET TO! I am about 2300 rpm I think.. (just what ic an remember

I have a dually 7.3 what could improve my mileage?!

 

oh sorry flat land driving...

70 mph...

no cargo...

never have had the ip or injectors rebuilt...

 

Flatbed,

What is your milage at 55 mph. Most guys I know usually report the best milage around 55 mph. By the time you get to 70 mph you could be easily down 3-4 mpg.

Simple things to increase milage:

• Replace your air filter
• Remove the soup bowl on the air cleaner lid
• Reduce you speed, drive 55 mph

More involved and costly things:

• Look into a second Overdrive Unit (from the 2300 rpm @ 70 mph it sound like you are already running a OD tranny)

 

Filters changed...
where I live the speed limit is 70 so can't really drive slower

Where and how to do a second overdrive unit?

 

Check out one of the following web sites:

https://www.ford-trucks.com/lc/lc.ph...vendors.com%2F

https://www.ford-trucks.com/lc/lc.ph...www.usgear.com

 

So 2300 RPM at 70 MPH says you have either an A4OD or ZF 5 with 4.10 gears and are running 235/85-16 tires.

First thoughts are this,
F350 dually with an 11 foot flatbed that is probably wider than the cab catches a lot of wind.
You said flat country so you also probably have to deal with a lot of constant winds while driving.
It is also a rather long wheelbase truck so that don't help either because it is rather heavy.
You say it takes a while to get to 70 so I guess you are having to be a bit heavy on the throttle just to maintain speed.

Are you seeing much black smoke?

Thinking on the screen,
I think you do have something that is getting weak in the IP or injector area, maybe both.
My truck is about the same as yours with the wide bed, I have 4x4 so my weight is probably close to yours but I do not have the overdrive and I have 3.55's, run 285/75-16 tires and a turbo. I am running 2600 RPM at 70 MPH.

I had my injectors pop tested and rebuilt. 4 were not spraying right and the pressure settings were low on all of them. The engine ran smoother and had more power but it did not change the mileage.

A couple of weeks later my IP started to change time, it kept advancing so I bought a reman unit from Ford for 431 dollars and installed it. Power increased more, mileage increased about 3 MPG while running 70 MPH. More if I drive slower.

If I were you, I think I would pull the IP and have it bench tested, if it is far out of spec I would swap it for a new or reman IP. If you want to have the injectors checked as well, if your prices are like they are here for fuel and the service work it will take 10,000 miles to recoup the cost of the IP and injectors replaced.

fuel 2.799 a gallon
DB 2 5028 IP 431 dollars exchange
injectors pop tested and rebuilt to turbo specs 289 dollars