Mich. Comp. Laws § 257.67a

Sec. 67a. (1) "Tandem axle assembly" means 2 axles spaced more than 3 feet 6 inches and less than 9 feet apart, 1 axle in front of the other and so attached to the vehicle wherein an attempt is made by connecting mechanism to distribute the weight equally between the 2 axles. (2) "Tank vehicle" means any commercial motor vehicle that is designed to transport any liquid or gaseous material within a tank or tanks having an individual rated capacity of more than 119 gallons and an aggregate rated capacity of 1,000 gallons or more that are either permanently or temporarily attached to the vehicle or the chassis. If a commercial motor vehicle transports 1 or more tanks manifested either as being empty or containing only residue, those tanks shall not be considered in determining whether the vehicle is a tank vehicle.