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Discussion Starter #1 (Edited)
New to the world of F-Series. Just bought my first pickup truck a couple weeks ago.

2003 F-250, XLT, Crew Cab, Short Bed, 7.3L, SRW, 4WD, Auto.
~250k miles
TS Performance 6 position chip. Stays in "stock" position/mode.
285/75/16 Toyo Open Country M/T
Aftermarket air intake. Not sure on brand but the filter is K&N. Restriction gauge indicates 65%.
Muffler delete.
Rest of truck is stock.

First thing I did was take a 3 hour road trip with my Scangauge II and some hand calculations on pure highway driving. Average at flat, 70 MPH was around 14 MPG.

  • Intermittent CEL that I cant retrieve with my scanner or my Scangauge II. Usually comes on when I start the truck cold but doesnt show up for every trip. Maybe 1 or 2 out of 4. Goes off typically after about 5 minutes of driving. Came on a couple times when I really got into the go pedal(which is extremely rare for me) and went back off after about a minute. SOLVED. Bad ICP sensor.
  • Lifted the oil dipstick. Noticed it was way overfilled. Dont know the graduations on the stick but it was a good 2-3 inches above the MAX FILL line in the hash marks. Oil was very clean and apparently is about 4k miles old. Corrected the level to about halfway in the hash range. Drove it for a few days and checked the oil again and it was at the max line. I was thinking I was leaking something into the crankcase. Decided a couple days ago to stop waiting for the "glowplug/wait to start" light to go off before cranking. Truck fires right up from an overnight cold without any hesitation. Ive decided to do this as a test to see if my oil level grows again. From what Ive read, the fuel pressure is much lower than oil pressure in these trucks so the only time you would get fuel migration into the oil is "key on, engine off" condition which is fuel pump on(60PSI), oil pressure 0 PSI. Im going to do this for about a week and check the oil. If the level doesnt move, Im going to repeat the process with the new variable of "wait to start" again and monitor for oil level change. Is that a good test to determine if my injectors are leaking into the oil? SOLVED. Oil level has remained steady.
  • Lifted the ICP sensor plug and it was oiled. Ordered an OEM sensor and replaced. MPG went from 13-14 @ 70 MPH to about 12 @ 70. WTH?!
Truck seems to run and idle just fine but this is my first Super Duty and my first diesel so I dont really trust myself to make that determination but nothing really stands out.

Aside from the possibility of leaking injectors which Ill hopefully be able to rule out in a couple weeks, where else should I look to improve on the MPGs? I can monitor a bunch of different parameters/sensors with my Scangauge II if it will help narrow down the cause.
 

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Discussion Starter #2
I just did some runs and I got -

26.8 mpg @ 45 mph
20.7 @ 55
16.1 @ 60
13.8 @ 65
13.1 @ 70

^^^***?!^^^
 
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Since there are a few axle ratios available, it would be more informative to post your RPMs rather than MPH.

The best fuel economy is found between 1,800-2,000 RPMs. (This is empty of flat ground).
You will need higher RPMs when working with a load or in hills.
 

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Did you replace the ICP with a Motorcraft part?
Did you have much oil residue in the ICP pigtail? If so, it is often required to change the pigtail as well as the ICP. Did you give the pigtail a blast of Brakleen when you installed the new ICP?

The ICP is not likely to be the reason for your fuel economy but these are good ICP replacement practices.

I have an identical truck as yours but I run Michelin Defender M&S in stock 265/75/16. You mentioned you are running 285/75/16.

I get an average of 17.0 combined city/highway in the summer. Ofcourse, colder climates will see a drop in fuel economy when the winter arrives.

The #1 reason for both good & bad fuel economy is driving style, but you are doing the right thing eliminating possible mechanical contributors.
 

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Since there are a few axle ratios available, it would be more informative to post your RPMs rather than MPH.

The best fuel economy is found between 1,800-2,000 RPMs. (This is empty of flat ground).
You will need higher RPMs when working with a load or in hills.
This is what Ive found with my recent numbers...

MPH - MPG - RPM
45 - 31.6 - 1220
55 - 23.7 - 1490
60 - 17.8 - 1630
65 - 16.1 - 1765
70 - 14.9 - 1900

You can see the mileage really drop off between 55-60 which just happens to be around 1600 RPM, where the 7.3s peak torque is made.

Did you replace the ICP with a Motorcraft part?
Did you have much oil residue in the ICP pigtail? If so, it is often required to change the pigtail as well as the ICP. Did you give the pigtail a blast of Brakleen when you installed the new ICP?

I have an identical truck as yours but I run Michelin Defender M&S in stock 265/75/16. You mentioned you are running 285/75/16.

I get an average of 17.0 combined city/highway in the summer. Ofcourse, colder climates will see a drop in fuel economy when the winter arrives.

The #1 reason for both good & bad fuel economy is driving style, but you are doing the right thing eliminating possible mechanical contributors.
Definitely OEM parts when I comes to sensors.

I had oil for sure but I hit it with some contact cleaner and dried it well.

Last tank averaged 16.7 MPG. Thats most highway miles.

Thats all Im trying to do is work through the process of deduction. Id like the truck to be in tip-top shape so Im tracking MPG changes with each improvement or part change with the hopes of reaching that magic 20 MPG that so many get to.
 

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If someone tells you he is averaging 20-mph then he is very likely telling stories. Especially with a 4R100. I do get 20+ highway with a ZF-6 over a 700-mile tankful of fuel but that is holding at 2,000-RPM and the last time was with a WY tailwind for half the miles. Combined city/highway with a ZF-6 is 18.8 based on my Fuelly hand measurements.

I drive like an old man with no hotrodding.. In fact, my buddies gave me a Slow Moving Vehicle orange triangle sign as a joke. But I use it often.
158152


I also use a ScanGauge-2 which reports both real-time MPG and also AVG mpg. I have mine dialed in tight but even if a person does not fine tune it, the SC-2 will still reflect what driving behaviors provide good economy and which ones guzzle fuel. I get better MPG by driving barefoot than with my heavy steel-toe boots on...haha.

Check out Fuelly.com if you aren’t already. You can see MPG by model year and engine size (fuel type).

Good that you use Motorcraft electrical parts. (Except for a GPR which should be White-Rogers).

I think you are on the right track with your RPM vs MPG and your idea of using fuel economy as an indicator of possible mechanical trouble starting is something I practice to.

One unknown for me is the effect of 285/75/16 combined with a 4R100 and how it affects performance. I have 265/75/16 on my 4R100.

But I do have 285/75/16 on my ZF-6 where I can control my shifting.

One other test I might be inclined to run if I had a tuner like yourself is to place it in the mildest tune and see how it affects MPG. Or even 100-HP tune. Just for kicks to see if the different shift points help compensate for your larger tires.

BTW, do you still have stock wheels? Steel or Aluminum? Its surprising how much heavier the steel wheels are.

Keep us posted as you make improvements.
 

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Also tire pressure. The difference between 50 and 65-psi is about 10% improved with the higher psi.

I have also found better fuel economy with about 300# in the bed if I am on the highway.

But the 7.3 PSD is a workhorse first and so atleast half the time I am more interested in its ability to work ;)than its fuel economy.
 

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If someone tells you he is averaging 20-mph then he is very likely telling stories. Especially with a 4R100. I do get 20+ highway with a ZF-6 over a 700-mile tankful of fuel but that is holding at 2,000-RPM and the last time was with a WY tailwind for half the miles. Combined city/highway with a ZF-6 is 18.8 based on my Fuelly hand measurements.
Ive been referring to realtime highway/cruising mileage only. And specifically at 70 MPH. Last tank I got about 16.8 AVG for the entire tank, mostly highway.

I drive like an old man with no hotrodding.. In fact, my buddies gave me a Slow Moving Vehicle orange triangle sign as a joke. But I use it often.
:LOL:
Might have to get me one of those.

I think you are on the right track with your RPM vs MPG and your idea of using fuel economy as an indicator of possible mechanical trouble starting is something I practice to.
Thats why I still keep and eye on the mileage on my other vehicles.

One unknown for me is the effect of 285/75/16 combined with a 4R100 and how it affects performance. I have 265/75/16 on my 4R100.
When it comes time to replace these tires Im certainly going with something skinnier. Im thinking 245/60-20 which should keep me at a similar OD with less width/rolling resistance. And the 16" rims just look too small on this truck. Out of proportion.

One other test I might be inclined to run if I had a tuner like yourself is to place it in the mildest tune and see how it affects MPG. Or even 100-HP tune. Just for kicks to see if the different shift points help compensate for your larger tires.
I keep forgetting that the tuners sometimes touch on the shift points. Anyone know if the TS Performance 6 position modifies shift points? Thats the one I have.

BTW, do you still have stock wheels? Steel or Aluminum? Its surprising how much heavier the steel wheels are.
OEM "bullethole" aluminum.


Also tire pressure. The difference between 50 and 65-psi is about 10% improved with the higher psi.

I have also found better fuel economy with about 300# in the bed if I am on the highway.
I did increase the PSI and it helped but I dont want to remove too much contact path with higher pressure because we get a ton of heavy rain where I live.

Why would the #300 help MPG?
 

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16.8 mpg at 70 mph is what I would call good MPG. Decreasing wind resistance tire drag and engine RPM are the keys to good MPG. What is you axle gear ratio?
DENNY
 

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16.8 mpg at 70 mph is what I would call good MPG. Decreasing wind resistance tire drag and engine RPM are the keys to good MPG. What is you axle gear ratio?
DENNY

No, no. Im getting just below 15MPG @ 70MPH right now. The 16.8MPG is over the entire tank at various speeds.

3.73
 

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OK, I see a trend. How are you getting all the numbers you are posting? I take it is off some type of gauge/scan. Go fill the tuck and drive a few hundred miles at a single speed, then refill and do some math. Repeat, at slower faster speed. It will take a while but good numbers do not come form any readout you truck has.
DENNY
 

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OK, I see a trend. How are you getting all the numbers you are posting? I take it is off some type of gauge/scan. Go fill the tuck and drive a few hundred miles at a single speed, then refill and do some math. Repeat, at slower faster speed. It will take a while but good numbers do not come form any readout you truck has.
DENNY
Calibrated Scangauge. I did do what you are referring to when I took a 3.5 hour road trip at 70 MPH. It was hand calculated and it matched the Scangauge.
 
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Just use a calibrated stubby pencil. That Scangauge is off when it comes to the lower speeds and the numbers you are posting are all over the board. Arctic is correct I would expect a 1 mpg hit just from the auto tranny. Get some kind of GPS app for your phone and make sure the speedometer is correct.
DENNY
 

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Why would the #300 help MPG?
Well there are two lines of thought on this.

I find a decrease in MPG with weight in bed when I am working/driving in town with many stopsigns or lights. I have read as much as expect to lose 1-MPG per 100#.

But on the highway, I think the extra weight (mass) adds to the momentum propelling the truck forward when buffeted by a headwind or a hill. Ofcourse, it would be a good argument that these same obstacles require the engine to work harder, ergo consume more fuel.

So I can’t provide any supporting evidence as to why the estimated 300# seems to help me out. It may just be my driving style.
 

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It may change the airflow over the bed and reduce drag. DENNY
 

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It may change the airflow over the bed and reduce drag. DENNY

Thats the only thing I could think as well. I would just get a roll away bed cover anyway.
 

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It may change the airflow over the bed and reduce drag. DENNY
In my experience, caps offer this advantage. Even the old lightweight aluminum caps from the 80’s used to improve the economy on my F150s. That continued with the heavier fiberglass caps as well.

But when I add weight like I described then I keep the profile low. I am pretty certain it is the weight itself that provides the improvement since it holds true even when I have the fiberlass cap on the bed.

In stop & go traffic, extra weight requires more energy (fuel) to overcome inertia but once the object (truck) is rolling then the weight contributes to the objects momentum.
 
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