321.466  Increased loading capacity — reregistration.
  1.  An increased gross weight registration may be obtained for any vehicle by payment of the difference between the annual fee for the higher gross weight and the amount of the fee for the gross weight at which it is registered.
  2.  During or after the seventh month of a current registration year, the owner of a motor truck, truck tractor, road tractor, semitrailer or trailer may, if the owner’s operation has not resulted in a conviction or action pending under this section, increase the gross weight of the vehicle to a higher gross weight classification by payment of one-twelfth of the difference between the annual fee for the higher gross weight and the amount of the fee for the gross weight at which it is registered, multiplied by the number of unexpired months of the registration year.
  3.  Upon conversion of a truck to a truck tractor or a truck tractor to a truck, an increased gross weight registration of the proper type may be obtained for the vehicle by payment, except as provided in section 321.106, of one-twelfth of the difference between the annual fee for the higher gross weight and the amount of the annual fee for the gross weight at which the vehicle is registered, multiplied by the number of unexpired months of the registration year from the date of the conversion.
  4.  The registered gross weight of a vehicle or combination of vehicles may also be increased by installing and using an auxiliary axle or axles, and the combined registered gross weight of the vehicle and auxiliary axle or axles shall determine the total registered gross weight. An auxiliary axle shall not be used to convert a single axle to a tandem axle unless equipped with a device to equalize the load carried by the single axle and the auxiliary axle when in tandem and when in motion or when standing, and the load transmitted to the highway by either the single axle or the auxiliary axle shall not exceed that permitted for a single axle, nor shall the load transmitted to the highway when in tandem and when in motion or when standing, exceed that permitted for a tandem axle.
  5.  A person shall not operate a motor truck, trailer, truck tractor, road tractor, semitrailer, or combination thereof, or any such vehicle equipped with a transferable auxiliary axle or axles, on the public highways with a gross weight exceeding the gross weight for which it is registered by more than five percent; provided, however, that any vehicle or vehicle combination referred to in this subsection, while carrying a load of raw farm products, soil fertilizers including ground limestone, raw dairy products, livestock, live poultry, or eggs, or a special truck, while carrying a load of distillers grains, may be operated with a gross weight of twenty-five percent in excess of the gross weight for which it is registered.
  6.  For the purposes of this section cracked or ground soybeans, sargo, corn, wheat, rye, oats, or other grain shall be deemed to be raw farm products, provided that such products are being directly delivered to a farm, from the place where the whole grain had been delivered from a farm for the purpose of cracking or grinding and immediate delivery to the farm to which such cracked or ground products are being delivered.
  7.  The truck operator shall have in the truck operator’s possession a receipt showing place of processing on the return trip.
[C24, 27, §4921; C31, 35, §4921-c1, -c2; C39, §5035.15; C46, 50, 54, 58, 62, 66, 71, 73, 75, 77, 79, 81, §321.466; 81 Acts, ch 110, §2]
82 Acts, ch 1062, §31, 32, 38; 90 Acts, ch 1233, §18; 2009 Acts, ch 113, §1
See also §321.460 
For applicable scheduled fines, see §805.8A, subsection 12, paragraph d