Ok, I appreciate the opinions, but my question is what tune will help economy the most.
Unfortunately, there is no hard data on this. Tuning varies, and so does driving style and vehicle configurations (mods, etc). Some folks have found that stock tuning works best for mileage, others have had "economy" tunes work best, and still others have used race tunes for better mileage. It really varies quite a bit.
Also, at what size exhaust does it start to drone on the 7.3.
Most systems are either 4" or 5". The drone can pop up on either setup, depending on what muffler you use, or if you don't use a muffler at all. Most standard 4" setups (the more popular and widely used ones) tend to have little or no drone.
I'm not a 7.3 guy, but I've owned and raced many diesels from 1999 up until last year(crazy racers to 10 second daily drivers to 11 second tow vehicles). I do understand wind resistance, and the theory that raising a vehicle will lower fuel economy. On a gasser, this is always true by my experience. Lifting a diesel can be a whole different animal. Diesels tend to be very rpm sensitive, as stated above. 2k or below is optimal. Over 2200 rpm, and economy starts really diving. A moderate lift of a vehicle will not always lower fuel mileage at a given speed on a diesel if a few inch taller tire is used that's not overly wide. Yes, aerodynomics are worse. Yes, a taller tire creates more rolling weight/drag. At times a diesel will overcome those negatives with the couple hundred rpms you lose by going a few inches larger.
Yes, the PSD is very RPM sensitive in regards to mileage. However, there's also a caveat concerning larger (taller) tires and/or lift.
Fueling is controlled by your right foot. Pressing the accelerator pedal the PCM reads the amount of fuel desired based on that pedal position measured in A/D counts. Let's say to maintain speed down the highway at 70 MPH on a stock EX, it takes 300 A/D counts of pedal to keep that speed. This results in a specific amount of mass fuel desired, and the PCM uses that input to determine injection pressure and fuel pulsewidth. So you have "X" amount of fuel being delivered to maintain that speed.
Now add larger tires and a lift. To maintain that 70 mph, yes you're running at lower RPM's. However, because of the additional drag, it now takes 400 A/D counts of pedal position to maintain that speed. It means that the PCM see's a higher demand of fuel desired in order to maintain that same speed (even at a lower RPM), and thus increases injection pressure and pulsewidth to match the fueling desired. So instead of "X" amount of fuel needed, you have a new value of "Y" amount of fuel needed.
In other words, while you might be running lower RPM's, you're actually fueling more to overcome the additional drag. This is just a crude example that I posted, but it's just to serve as an example of how the PSD reacts. To overcome additional drag to maintain the same speed, it requires more fueling.
Of course, it's all dependent upon the actual increase in drag that is incurred with larger tires and/or a lift. If you were to get taller tires, but without increasing the width of the tire and without the need of a lift, you could very well increase mileage, as the rolling resistance of the taller tire might not be enough of a drag to warrant too much additional fueling. But bundling taller tires along with a lift, you're adding even more drag, and the chances of you actually getting better mileage will significantly decrease.... and the most likely outcome would be worse mileage... especially at the highway speeds you posted earlier.
Granted, these results were with a few of my past dodges. My 2003 LB7 back in the day turned in 18 on the highway with the stock tiny tires. Approximately the same on 34'' tires, and over 16 on 39's. My dodges were heavily modded as well. The LB7 was just a 13 second truck. Peoples lifting experience and theories are kind of like gear ratios of drag trucks. Some still have a more gasser mentality that lower gears help in drag racing higher power diesel trucks. I've found out by experience that even going from a 3.73 to a 3.42 gear ratio I brought my time down around a tenth and a half in the quarter. It also let me hook up better off the line making et's more consistant. I'm sorry for writing so much. i just wanted to clarify that I'm not a diesel new guy. If I were going for exteme fuel economy, I'd drop in high gears, lower it, and install lightweight wheels and a light narrow tire. I'm not that worried about it.. haha.. I just don't know the latest and greatest 7.3 tunes.
Just speaking from personal experience with my own truck.... I did buy it with the lift already installed. However, I knew the former owner, and I drove the truck regularly when it was still stock. At 65 mph on the highway, 18 MPG was pretty easy to hit. He then put the lift on, and it was routinely getting 16-17 mpg on the highway at the same speeds. This was with 315/75r16 tires at that time, and still with the stock 3.73 gears.
I was able to reach 18 mpg... but it involved getting a chip, a bed cover, and driving for 800 miles one way keeping speeds at around 60 MPH. So I had to drive slower and modify the truck in order to reach the same mileage that it saw when it was at stock height with stock tires. All of this was still with those same 315 tires, and stock 3.73 gears. Now I'm running even bigger and wider tires, and have 4.30 gears. 18 MPG is something I just haven't been able to duplicate at all now.
Anyway, just trying to help. I would hate for you to invest in a lift and tires only to find out that your mileage drops without anyone warning you.
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