225 ILCS 470/41
No person shall operate, upon the streets or highways of this State any vehicle tank used for commercial purposes unless such tank either is equipped with a meter or other device for measuring deliveries from the tank or has been calibrated for capacity and sealed by the Director . When a vehicle tank has been calibrated for capacity by the Director , he shall issue to the owner or operator a certificate of calibration in which is shown the calibrated capacity of each compartment. A copy of this certificate shall accompany the vehicle tank at all times or kept on file available for examination either at the plant out of which the vehicle tank is operated or at a regional or principal Illinois office of the owner of the vehicle tank. Each compartment of a vehicle tank shall be marked with a statement of its capacity as defined by its indicator, located in a conspicuous place in letters and figures not less than one inch in height. Enforcement of this Section is reserved to the Director or to the sealer in a city having a population of 200,000 or greater according to the latest official United States census.
225 ILCS 470/41