26 C.F.R. § 48.4041-7
Tax applies to all taxable liquid fuel sold for use or used as a fuel in the motor which is used to propel a diesel-powered vehicle or in the motor used to propel a motor vehicle, motorboat, or aircraft, even though the motor is also used for a purpose other than the propulsion of the vehicle, motorboat, or aircraft. Thus, if the motor of a diesel-powered highway vehicle or a motorboat operates special equipment by means of a power take-off or power transfer, tax applies to all taxable liquid fuel sold for this use or so used, whether or not the special equipment is mounted on the vehicle or boat. For example, tax applies to diesel fuel sold to operate the mixing unit on a concrete mixer truck if the mixing unit is operated by means of a power take-off from the motor of the vehicle. Similarly, tax applies to all taxable liquid fuel sold for use or used in a motor propelling a fuel oil truck even though the same motor is used to operate the pump (whether or not mounted on the truck) for discharging the fuel into customers' storage tanks. However, tax does not apply to liquid fuel sold for use or used in a separate motor to operate special equipment (whether or not the equipment is mounted on the vehicle). If the taxable liquid fuel used in a separate motor is drawn from the same tank as the one which supplies fuel for the propulsion of the vehicle, a reasonable determination of the quantity of taxable liquid fuel used in such separate motor or during such period is acceptable for purposes of application of the tax. This determination must be based, however, on the operating experience of the person using the taxable liquid fuel, and the taxpayer must maintain records which support the allocation used. Devices to measure the number of miles the vehicle has traveled, such as hubometers, may be used in making a preliminary determination of the number of gallons of fuel used to propel the vehicle. In order to make a final determination of the number of gallons of fuel used to propel the vehicle, there must be added to this preliminary determination the amount of fuel consumed while idling or warming up the motor preparatory to propelling the vehicle.
26 C.F.R. §48.4041-7