Axles of a motor vehicle, trailer, or semitrailer which are less than forty inches apart center to center shall be considered as a single axle for the purpose of determining permissible gross weight under section 321.463 . [C31, 35, §5067-d3; C39, § 5035.08; C46, 50, 54, 58, 62, 66, 71, 73, 75, 77, 79, 81, §321.459 ] Referred to in §321E.2