All states are front for tractors. Even some 1 tons with fifth wheels or goosenecks qualify for the plate to be on the front in some states like Tennessee. It optional in some states and you can put the plate on the rear but Michigan requires the plate to be on the front of a tractor. The law is below for your reading pleasure.
257.225 Registration plate; attachment to vehicle; legibility; color; distinctive registration
plates; name plate, insignia, or advertising device prohibited; violation as civil infraction.
Sec. 225. (1) A registration plate issued for a vehicle shall be attached to the rear of the vehicle. Except
that a registration plate issued for a truck tractor or road tractor shall be attached to the front of that vehicle.
(2) A registration plate shall at all times be securely fastened in a horizontal position to the vehicle for
which the plate is issued so as to prevent the plate from swinging. The plate shall be attached at a height of
not less than 12 inches from the ground, measured from the bottom of the plate, in a place and position which
is clearly visible. The plate shall be maintained free from foreign materials that obscure or partially obscure
the registration information, and in a clearly legible condition.
(3) A registration plate or the expiration tab on the registration plate shall be of a different color designated
by the secretary of state with a marked contrast between the color of the registration plate and the numerals or
letters on the plate. The secretary of state may provide a distinctive registration plate as a replacement for a
standard plate. To honor a special or historical event, the secretary of state may provide a commemorative
plate as a replacement for a standard plate.
(4) A person shall not attach a name plate, insignia, or advertising device to a motor vehicle registration
plate in a manner which obscures or partially obscures the registration information.
(5) A person shall not operate a motor vehicle which has a name plate, insignia, or advertising device
attached to a motor vehicle registration plate in a manner which obscures or partially obscures the registration
information.
(6) A person who violates this section is responsible for a civil infraction.