I just bought a new 2017 Dodge RAM 2500 (w/DEF) replace my 2008 Dodge RAM 3500 (non-DEF). I need to track cost per mile and adding this to the fill up page would be a nice feature that I requested. In the meantime, I Have created a separate vehicle call Def-Dodge and I enter only my DEF information there (mileage, gallons, cost)
To see cost per mile I just go to statistics, search and select both dodge vehicles and the accurate cost per mile will show.
Fuelly needs to isolate this category or make a new one that doesn't skew the mileage results for other fuels for the same vehicle.
I track DEF as a vehicle using fuel Category: Other Type: DEF that I added and it seems many others track DEEF but are using Category Gas or Diesel.
Doing this doesn't cause my data to throw off MPG for other Diesel or Gas, it only seems to affect the "all" group. Sadly there are obviously others that are doing it (wrong) and using a vehicle to track DEF but must be selecting Gas or Diesel. I can see the 350+MPG records of others in Fuelly even filtered on several of the Diesel or Gas selections.