Is a diesel engine better if it uses more DEF? Maybe not
The DEF is used to take the NOX out of the exhaust. The NOX is an oxide of nitrogen, which means it comes from burning the air, and that requires a very high combustion temperature. But, you get the very high combustion temperature when the engine runs lean. When the engine runs lean, it uses less fuel. Thus, there is a case to be made that using a lot of DEF means that your engine is burning fuel more efficiently.
To take it to the other extreme, an alternative to using DEF is Exhaust Gas Recirculation (EGR). Cummins has a white paper explaining that they considered it but found they couldn't get good fuel mileage with it. Of course, with EGR, they use no DEF. So, Cummins uses DEF to get better fuel mileage.
Similarly, in the Volkswagen emissions defeat scandal, they have considered using more EGR and stuff like that, and fuel mileage will collapse. They can't afford to retrofit a DEF solution.
I'd have to look at my records, but my Cummins ISB 6.7 seems to use about 6 gallons of DEF for 2000 miles, but it gets fantastic fuel mileage – 10 mpg (US gallons) at 65 mph with a diesel pusher motorhome.
My biggest hassle with DEF is that there is a roof very close to my DEF tank, which makes it very hard to put it in from the jugs. I have to keep an eye out for bulk DEF dealers. Unfortunately, that means a lot of Flying J or Pilot, where the fuel is expensive.
Gordon Sick, Calgary
2015 Berkshire 34QS
2005 Acura EL (aka Honda Civic)