detroit 14l terrible mpg

So your fuel isn't being eaten by the EGR system - say a valve sticking open. Black smoke is a sign of unburnt fuel.

How about a leak in the charge air system? Have you tested that for leaks yet?

 

I think that would certainly be what you would call poor fuel milage!! I know im gonna get bashed for this but, here goes....I really don't think gearing or rpm has alot to do with it!! I have had trucks with cat,cummins,and 12.7 Detroits. They've had almost every combination of gears and transmissions in each one and all got within a tenth or two of the same milage! they have from 3.42 ratio all the way down to 4.56! I have not got any better than 4.80 mpg!!!! I haul heavy equipment all the time never anything else. I'm sure if i was a lil lighter I could get better!!!

 

Well... it doesn't sound like you have a mechanical issue. Getting the overhead done would get that eliminated, if you haven't already done that.

Sounds like you may have to finesse some mileage out of it through technique.

 

I don't have any productive input, and I don't have clue what could be going on, but I don't think there's any way this is entirely a technique issue. Running legal loads at a reasonable speed, you shouldn't be able to to run this thing hard enough to burn 4mpg worth of fuel. You'd have to be driving into a 50mph headwind with a parachute on the back of the trailer to make that thing burn that much fuel if everything was running right. I'm assuming by "tuneup" you mean overhead.

 

Don't listen to these jokers that say: driver better, blah, blah, shift better, blah , blah !

There is no way, EVEN IF YOU TRIED ON PURPOSE, to get 4 mpg driving in the south (I assume it's the south since you live in MS) or anywhere else for that matter unless you haul OD loads over the hills in the west or something into a 30 mph wind.

Is the other truck you got running 5.5 avg have the same setup as this one ?