26 CFR § 48.4061(a)-4 - Parts or accessories sold on or in connection with chasis, bodies, etc.
(a) In general. The tax attaches in respect of parts or accessories for articles specified in section 4061(a) sold on or in connection therewith or with the sale thereof at the rate applicable to the sale of the basic article. The tax attaches in such case whether or not the parts or accessories are billed separately. For the tax applicable to parts or accessories which are not sold on or in connection with the sale of a taxable chassis, body, or tractor, see § 48.4061(b)-1.
(b) Essential equipment. If taxable chassis, bodies, or tractors are sold by the manufacturer, producer, or importer without parts or accessories which are considered equipment essential for the operation or appearance of such articles, the sale of such parts or accessories will be considered, in the absence of evidence to the contrary, to have been made in connection with the sale of the basic article even though they are shipped separately at the same time or on a different date. For example, if a manufacturer sells to any person a chassis and the bumpers for such chassis, or sells a taxable tractor and the fifth wheel and attachments, the tax applies to such parts or accessories at the same rate as on the chassis or tractor regardless of the method of billing or the time at which the shipments were made.