Diesel Exhaust Fluid (DEF) Consumption and Range
Why a DEF tank spans several fuel fills, and never runs it dry: DEF is metered into the SCR aftertreatment at only about 2-3% of diesel, so def_used = diesel x dose/100 and a full DEF tank covers diesel = def_tank / (dose/100). A 13 gal DEF tank at a 2.5% dose covers 520 gal of diesel = 3,380 mi at 6.5 mpg on one fill (200 gal of diesel burns just 5 gal of DEF); a hard-pulling 3.0% dose shortens it to 433 gal = 2,817 mi. Plan DEF per fuel stop and you haul jugs you do not need or run it dry -- and empty DEF forces an ECU derate to ~5 mph limp-home until refilled. DEF freezes at ~12 F. A planning estimate; the OEM and DEF quality govern.
Formula and source
diesel = diesel_gal > 0 ? diesel_gal : trip_miles / mpg; def_used = diesel x dose/100; diesel_per_def_tank = def_tank / (dose/100); range = diesel_per_def_tank x mpg.
DEF consumption and range model (SCR aftertreatment; the ISO 22241 diesel-exhaust-fluid spec), by name; the OEM and the DEF quality govern the metered rate.
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