Since I track my mileage every tanks via my phone I decided to do an experiment. I had read a number of posts by people with similar vehicles that stated the best way to increase mileage is to slow down. I did this and here are the numbers I found....

Vehicle: 1988 E250 7.3 IDI NA, 3.54 gears, c6+Doug Nash O/D

Procedure: I tracked my mileage for 4 tanks during my normal weekly commute. My commute is 60% highway and 40% city. Terrain is slow rolling hills. For each tank I set my cruise when I was on the highway for a different speed. I started at 75-80mph and dropped each tank in 5mph increments. I filled up at the same gas station on the same pump for each tank. For consistency I also put the DN in over drive and just left it. This made it pretty doggy around town since it gave me the equivalent of having 2.83 gears.

Tank #1, 75-80mph: I drove 387 miles using 27.851 gallons of fuel. Netting 13.90mpg

Tank #2, 70mph: I drove 425 miles using 27.784 gallons of fuel. Netting 15.30mpg

Tank #3, 65mph: I drove 452 miles using 28.733 gallons of fuel. Netting 15.73mpg. This tank could be slightly skewed. I was having charging issues so I let it idle during stops instead of risking it not starting due to a low battery. Idle time was about 1 hr total. Some brief searching said I would use about .5 gallons an hour at idle. Removing this would give a mileage number of 16.01mpg. It was also raining consistently throughout this week.

Tank #4, 60mph: I drove 464 miles using 26.898 gallons of fuel. Netting 17.25mpg. I did accidentally set the cruise to 65mph for one 25 mile highway trip, but this would only effect the results by about .05mpg.

Conclusion: While there are a ton of variables and one tank sampling per speed does not provide the most accurate results, it does seem to show that reducing your speed does seem to have a fairly large positive affect on mileage. Mileage showed a 24% increase between tank #1 and tank #4.

Other thoughts: I would be curious to see if dropping to 55mph would continue the trend, unfortunately I couldn't bring myself to slow down that much. Our speed limit in the area is 70mph with most people going 75-80mph. Going that much slower than the traffic flow would drive me crazy. I also wonder if having the OD engaged during city driving had a negative affect on my numbers. It definitely took more throttle input to get up to speed and my EGT seemed to be higher doing it vs with the OD off. Once cruising my EGT were significantly lower with the OD on. I also did not check my tire pressures, but since I just had them rotated and balanced I would assume they set them to the recommended stock pressure. Increasing them to 80psi may reduce rolling resistance and net a small gain.

 

Your van is in good running condition, before, I had a timing meter and before the e4od in my 88. speed did not affect my mileage. it did a consistent 11mpg with a low stall converter and 8mpg with the mid stall that the trans shop installed on that last c6. I tried 65, then dropped to 55mph.
Installing the e4od with out tuning got me to 14, then tuning it got me up to 20mpg at 65
My point is if your mileage is consistently around ten, unless you have low gears, there is something wrong with your idi.
If your still in the experimenting mood I would do your 60mph test with your overdrive locked out. you might be surprised what happends

 

Do you mean running 60mph with the overdrive off the whole time?

On the route that I take to work I was getting 14-14.5mpg consistently with one tank at 15.4mpg. This is with a low stall converter and the cruise set at 65mph. Best I ever got pre overdrive was 16.04mpg once on an all highway trip on flat ground with the cruise set at 65-67mph. Since the install I have been consistently in the 15.25-16.25mpg range during my daily commute with the cruise set at 65mph. Gotten over 17mpg twice in the past month and hit 18.4mpg once on an all highway flat area (even with 10 people in the van and a small trailer full of luggage).

 

Dropping down to 55 will also increase your fuel mileage, because around 59 or 60mph, you hit a "wind wall", and once you hit that wall, thats where mpg drop the most, cause your engine is working harder to keep you moving through that wall.

Theres another "wall" at 180mph if I remember correctly.

 

I believe that's terminal velocity, which can only be reached by dropping one of our trucks out of an airplane. Not recommended.

 

I had a 1974 Mercedes 220D. Right in the owner's manual they printed a MPH-to-MPG graph. The MB data, and my experience, showed the MPG kept getting better well below 55 MPH. I owned the car in the 55 mph era, and it was easier then to keep it slow.

Roy

 

I was thinking that with the od on all the time you might not be in the power band of the engine. My 95 and a friends ranger gets better mileage at 65 than 55, because the engines are not in the power band. It would probably help to drop a gear, but I rather drive 65. I was guessing that your rpms would be 18-1900 with od on and 21-2300 with it off.

 

if you could drive around in overdrive (without lugging w/ manual) at just where the trucks shifts into OD (and remains there w/ auto without downshifting) that should be the approx peak zone.for chip truck it would be 40MPH.for log truck it would be about 30MPH lol.low rpms and low headwind is the key.......not exactly fun,but that's how ya get to point a to b as cheaply as possible.

 

I'll point out that with a fully locked up system(i.e. manual, or perhaps E4OD) and good gearing, you can easily hit decent mileage in the ~75MPH range.
Last year, with my rig(zf-5, GVOD, 4.10s ext-cab 88, Banks turbo), I did a 1K-mile round trip on mostly flat highway; I averaged somewhere in the 17MPG range(Hit 18MPG on the all-highway trip over, averaged 16.5 or so for the lower-speed commuting at the end of my trip and the trip back.
This year, I took basically the same trip, but with a different engine(I think it's a bit retarded, actually, and it's got a loud ticking problem) and averaged 16.5 for the entire trip. This time I was a bit faster than last time, going 73-77 due to not having a working cruise control.
I suspect that when I take the same trip next year, if I've got a /good/, well-tuned engine in it for once I should hit 18 easily.

On my normal weekly commute, which is 55 miles, of which most is 55-60 speed limit, with a couple of city sections, and I've been doing tank-to-tank averages of 14-16.5. Hauling a full trailer(in this case, flatbed trailer with 4' sides), I get in the 12-14 range, currently.

 

Wind resistance is proportional to speed squared. So MPG should go down roughly relative to speed squared.

 

That would be true if wind resistance were the only factor in fuel economy. It's not.

 

My diesel is a service truck with a full load of tools and a T-19 with 3.55s. I average 18.3mpg at 55 and it drops to about 16.8 at 65. I drive it fairly nice except I leave a bit of a haze up hills and I am in North Idaho so we have a few of them. Hopefully, I will soon be able to tell you how it does with a turbo!!

 

Mileage drops as the fun factor (read: turbo) increases!!!

 

Not really... It only affects things if you are running too rich(I.e. flooring it and smoking). actually, in a lot of cases, it /increases/ economy a bit, due to the engine not being starved for air above 2000RPM.
Honestly, I didn't see much of a difference economy wise before and after... but I sure as heck started putting more foot into it afterwards, driving it harder(so any gains were canceled out, except on nice long highway cruising).

 

i noticed a pretty big dip from n/a to turbo.about like the srw to drw dip.not pretty but it sure gave the old gal a good kick in the pants.