I have a 2015 freightliner cascadia dd15 with manual Eaton 10 pulling a 48′ spread axle flatbed and getting an average 5mpg at 70mph (10tth gear runs at 15.2rpm and 9th runs at 20.2rpm) avg gross 78,500lbs. I do leave my truck on over night but only if it really cold or really hot. I’ve been reading alot of forums talking about O/O getting between 6.5mpg and 7.2mpg (there 9th gear is at 13rpm) so I dropped my speed down to 60mph avg 5.6mpg (9th gear runs at 17.5rpm and 10th gear runs at 13rpm) how do I get my rpms and mpg to what they are getting.
Sounds like you have very low rear ends like around 390s or 370s. The only way you’re going to get fuel mileage is slow down to about 65 around 1300 rpm. Only other option is change out the rear ends but that is expensive and may only get you a few 10ths.