ISX450.......unrealistic expectations?

[Moose]

450/1650 with the proper gearing is more than enough to run the weights you describe at 70 comfortably and should be getting 5.5-6.5mpg even mandated. IMO it's one of the better ratings for this engine. As stated above either you have something mechanically wrong on the engine, EGR and aftertreatment or it is staying in a silent derate because it sees something it doesn't like.

I haul aggregates grossing 90k typical but sometimes over and I had no problems with mine doing that. You need to decide if you want to leave the mandate intact or remove it then move forward with repairs.