35 ILCS 505/13a.2
Each motor carrier shall keep records which accurately reflect the type and number of gallons of motor fuel consumed, the number of miles traveled with each type of fuel on the highways of each jurisdiction and the highways of Illinois, the type and number of gallons of tax paid fuel purchased in this State, and every jurisdiction, and the number of miles traveled and the amount of fuel consumed on the highways of this State and every jurisdiction. Licensees shall preserve the records for a period of 4 years from the due date of their returns or the date filed, whichever is later. In the absence of such records, the Department shall presume that one gallon of fuel is used for each 4.0 miles traveled in this State. Every authorized agent of the Department shall have power to make any reasonable investigations to prevent avoidance of the tax imposed by Section 13a hereof.
Failure to provide records demanded for the purpose of audit extends the statute of limitations until the records are provided.
35 ILCS 505/13a.2