Freightliner classic preemissions pulling a van mpg?

I have a 2017 freightliner Coronado glider kit. My MPG about 7.5 per gallon running in the southeast about 62 mph to 64 mph ..... i’m running a 1999 12.7 Detroit diesel

 

6 mpg on average sound about right. I pulled reefer with a classic Freightliner ( year 1998) and it had Detroit engine - heavier loads than dry van - and was getting between 5.5 - and 6. Then I was pulling reefers with a Freightliner Centric (year 1999) for quite some time with Cummins N14 and the results were the same. The runs were mostly to the West Coast from Midwest and back so that's that added to the equation.

Nowadays, I pull a dry van with Volvo that has Cummins ISX and hardly ever go beyond 6.5 mpg as an annual average.

P.S. I should have mentioned, back then in early 2000s Diesel was for $1.25 so riding 75 - 80 mph through Nebraska and Wyoming was a common practice....

 

I had a 2001 Classic condo roof with a 565 ISX 4.10 rears and 11R24.5 tires. I got 5.88 for an overall average. I usually ran 62-64 mph

 

All of the Detroit 12.7 I had averaged 5.8 when we ran a trucking company. I did have a few drivers that could get better but they also were not go getters. I had one guy that was both and he got 8 MPG in the same truck that everyone else got 5.8. I almost cried when he left us. : )

 

I can get 5.5 without really trying in my w9/n14 in the north east, I’m sure you could squeak 6 out on the flat lands

 

I got two Detroits in Volvo trucks. One is tuned with bigger turbo, has about 525hp. The last trip I did in it was Detroit to Knoxville, then PA and back to Detroit. 43klb the whole time, 70mph the whole time (where it was permitted of course), got 6.3 mpg average manually calculated from tank to tank.
When light that truck does 7-7.5

I drove the second truck only for like 10k miles, because I just bought it, but it has not gotten less than 6.5mpg yet in those miles.
On my Volvo's with N14 it's a little worse, 6.2-6.5., but never below 6, unless there is like 15mph wind out there

 

Ok, first a 2006 is not pre emissions. It is a pre DPF.
Second if you are concerned about fuel mileage (which is a good thing) why are you considering a classic/hood?
Third a loose nut behind the wheel can waste fuel in any truck regardless of the brand or emissions level of the engine. This is why mega trucks are set up they they are, to reduce the impact the loose nut has on fuel consumption.