An autonomous auxiliary diesel exhaust fluid "DEF" supply system for supplying DEF to an onboard DEF tank in a diesel engine, comprising: an auxiliary DEF tank; an auxiliary DEF supply line configured for fluid communication between the auxiliary DEF tank and the onboard DEF tank; a pump configured to force DEF through the auxiliary DEF supply line; and a controller electrically coupled to the pump; wherein the controller is configured to command the pump responsive to DEF level signals received from an engine control module of the diesel engine; characterized in that: the controller is configured to execute a routine encoded in software for calculating onboard DEF tank volume; and wherein the routine executed by the controller comprises: receiving a low DEF level signal from the engine control module that indicates a low level of DEF in the onboard DEF tank; running the pump for a preset period of time representing a single fill cycle selected to deliver to the onboard DEF tank a volume of DEF that is less than a total volume of the onboard DEF tank; receiving an actual DEF level signal from the engine control module that indicates a higher level of DEF in the onboard DEF tank; calculating the total volume of the onboard DEF tank based on the low level of DEF, the delivered volume of DEF, and the higher level of DEF; and running a subsequent number of fill cycles to deliver the maximum volume of DEF to the onboard DEF tank without exceeding the total volume. an auxiliary DEF tank; an auxiliary DEF supply line configured for fluid communication between the auxiliary DEF tank and the onboard DEF tank; a pump configured to force DEF through the auxiliary DEF supply line; and a controller electrically coupled to the pump; wherein the controller is configured to command the pump responsive to DEF level signals received from an engine control module of the diesel engine; characterized in that: the controller is configured to execute a routine encoded in software for calculating onboard DEF tank volume; and wherein the routine executed by the controller comprises: receiving a low DEF level signal from the engine control module that indicates a low level of DEF in the onboard DEF tank; running the pump for a preset period of time representing a single fill cycle selected to deliver to the onboard DEF tank a volume of DEF that is less than a total volume of the onboard DEF tank; receiving an actual DEF level signal from the engine control module that indicates a higher level of DEF in the onboard DEF tank; calculating the total volume of the onboard DEF tank based on the low level of DEF, the delivered volume of DEF, and the higher level of DEF; and running a subsequent number of fill cycles to deliver the maximum volume of DEF to the onboard DEF tank without exceeding the total volume. the controller is configured to execute a routine encoded in software for calculating onboard DEF tank volume; and wherein the routine executed by the controller comprises: receiving a low DEF level signal from the engine control module that indicates a low level of DEF in the onboard DEF tank; running the pump for a preset period of time representing a single fill cycle selected to deliver to the onboard DEF tank a volume of DEF that is less than a total volume of the onboard DEF tank; receiving an actual DEF level signal from the engine control module that indicates a higher level of DEF in the onboard DEF tank; calculating the total volume of the onboard DEF tank based on the low level of DEF, the delivered volume of DEF, and the higher level of DEF; and running a subsequent number of fill cycles to deliver the maximum volume of DEF to the onboard DEF tank without exceeding the total volume. receiving a low DEF level signal from the engine control module that indicates a low level of DEF in the onboard DEF tank; running the pump for a preset period of time representing a single fill cycle selected to deliver to the onboard DEF tank a volume of DEF that is less than a total volume of the onboard DEF tank; receiving an actual DEF level signal from the engine control module that indicates a higher level of DEF in the onboard DEF tank; calculating the total volume of the onboard DEF tank based on the low level of DEF, the delivered volume of DEF, and the higher level of DEF; and running a subsequent number of fill cycles to deliver the maximum volume of DEF to the onboard DEF tank without exceeding the total volume.