26 C.F.R. § 48.6420(a)-2
Payment may be claimed under section 6420 only in respect of gasoline used on a farm in the United States for farming purposes. No payment is allowable under section 6420 with respect to gasoline used for nonfarming purposes, or gasoline used off a farm, regardless of the nature of such use. If a vehicle or other equipment is used both on a farm and off the farm, or if it is used on a farm both for farming and nonfarming purposes, payment is allowable only with respect to that portion of the gasoline which was "used on a farm for farming purposes" as defined in paragraph (a) of § 48.6420(c)-1 . The type of equipment or vehicle and whether or not it is registered for highway use is immaterial. However, the actual use of the equipment or vehicle and place where it is used are material. For example, if a truck used on a farm for farming purposes is also used on the highways (even though in connection with operating the farm), the gasoline used in operating the truck on the highways is not to be taken into account in computing the payment for which a claim is filed, since such gasoline was used off the farm.
26 C.F.R. §48.6420(a)-2