The EGR and DPF systems used in diesel trucks cause (or caused, it’s been a while since I last looked it up) a big reduction in fuel mileage. I think it was a 2 or 3 MPG reduction.Doesn’t sound like much, but it adds up when you are running 200,000+ miles a year per truck. With the system running I believe the average fuel mileage for the trucks in our company is around 6 to 8 MPG depending on the route.
Additionally the systems are expensive as all get out to repair and maintain. When the DPF and DEF systems on my truck went out, the truck was down for 3 months waiting on parts, and from what I heard from our mechanic, the final repair bill was around $15,000USD.
That said, the fines for bypassing the emission systems are big enough that it’s really not worth it, but some owner operators don’t realize it as many don’t run their trucks like the businesses they are. They just know this is expensive as hell and they can reduce the cost by bypassing them.
Don’t quote me on it, but I believe that the emissions tech is efficient enough that even with the increased fuel consumption there is an overall reduction in emissions across the board. That was my understanding when the tech was first being fazed in back in the mid to late ‘00’s. Whether that was true or just propaganda, don’t know. I just knew that full compliance was required to run in California and a few other states.