Ok, look at mi sig, truck is awesome empty, 21mpg at 65mph highway, and 19mpg at 75mph highway

Yesterday I put a 900 Interstate miles trip with a load and I had a big surprise, with a 30ft flatbed gooseneck empty (approx 2500lbs) I was getting 11.8mpg at 70mph, coming back with a Ford E250 van on top of trailer (approx 8000lbs total) I was getting 12.2mpg at 65mph .....

What the heck..... I thought the 5 speed would get better mileage but I did better with my '99 24 valve auto?

Now here comes the VERY interesting part:

dropped trailer and drive truck for 300 miles empty (interstate), on the way back I lost 5th gear (probably the nut deal) so I went for approx 100 miles at 62mph in 4th gear at 2500rpm.
Trcuk did better mileage (approx 22mpg) this way.....no kidding.

What is the problem ? I am ready to go beck to stock and try 370 injectors, or what do I have to adjust my pump to ?
Would be an appointment to Diesel tech be worth it ?

 

wind resistance would be a big factor, take a look at your post, in every situation you mentioned you got better milage going slower. What gear were you hauling in? If you were low on RPM its worse and while towing your air fuel control has a big effect, every time you add the least bit of load to your engine you dump fuel before the proper amount of air needed. Even if you don't see black smoke out the pipe there is a constant hase. I don't know just guessing.

 

I see your point and I agree to it, but wind direction is not a factor, I think to knock down mileage by 50% I had to have a at least 60mph wind in the front, which would have instead helped on the way back.

In any case there was no wind anywhere, I think my powerband is a little high for what I like, I am dumping too much fuel while air is not there yet, dropping 1 gear made me gain rpms but my boost psi lowered and so my EGT, this means motor was more efficient at 62mph @ 2500rpm than 65mph @ 1800rpm

I will hookup trailer again today and try to travel few miles in 4th gear at 2500rpm and see what mileage I get.
It could be I might have to drop 1 gear while towing or tow at a faster speed like 75mph or 80mph to reach the "sweet spot" in my motor.

I am also sliding plate back a little to see it this changes.

 

When I lost 5th gear I got the best fuel milage I have ever got with a diesel.

 

For what it's worth I tow 10m lbs. plus, several times a year and get 12 to 13.5 mpg depending on terrain. I keep it under 65 mph. I think there is a line, at some speed, that really makes a differance in mpg numbers. A change of just a few mph can make a nice differance.

 

Read Cummins secrets to better mpgs
http://72.14.203.104/search?q=cache:...s&ct=clnk&cd=1

 

When we got our 5th wheel I experimented with speed and found that keeping the rpm at or above 2000 was the secret to pulling and mileage. Below 2000 the engine seemed to be lugging and needed a lot more throttle to maintain speed. Towing between 65 - 70 mph keeps the rpm between 2000 - 2250 and gives mileage figures of 12 -14 mpg. It's possible that you were seeing that effect when towing in fourth gear.