united-states motor-vehicle license-plates
Legal to drive a car without front tag out of registered state?
<p>In the U.S. some states do not require cars to display license plates on the front of a vehicle, but a majority require plates on both the front and rear, as shown in the map below.</p>

<p><a href="https://i.stack.imgur.com/uwSHe.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uwSHe.png" alt="License Plate requirements by state"></a></p>

<p>My car is registered in Pennsylvania, so I only have a plate on the rear.  AFAIK, I can drive it into any state that requires front plates without violating a law, because I comply with the registration state's law.</p>

<p>But what if my car is registered in a state that requires a front plate (e.g., California) and I drive it in <em>any other state</em> without displaying a front license plate.  Is there any state in which I could be cited for a legal infraction?</p>

<p>At least some two-plate states suggest that if you're registered in a two-plate state you must display both plates within their borders. And some even suggest you must have two plates even if your state only issues one, though this scenario would probably not withstand federal scrutiny. In <a href="https://app.leg.wa.gov/rcw/default.aspx?cite=46.16A.200" rel="nofollow noreferrer">Washington</a>, </p>

<blockquote>
  <p>License plates must be: (i) Attached conspicuously at the front and
  rear of each vehicle if two license plates have been issued; (ii)
  Attached to the rear of the vehicle if one license plate has been
  issued.</p>
</blockquote>

<p>If it was issued, you must display it. <a href="https://leginfo.legislature.ca.gov/faces/codes_displayText.xhtml?lawCode=VEH&amp;division=3.&amp;title=&amp;part=&amp;chapter=1.&amp;article=9." rel="nofollow noreferrer">California law</a> says </p>

<blockquote>
  <p>(a) When two license plates are issued by the department for use upon
  a vehicle, they shall be attached to the vehicle for which they were
  issued, one in the front and the other in the rear. (b) When only one
  license plate is issued for use upon a vehicle, it shall be attached
  to the rear thereof, unless the license plate is issued for use upon a
  truck tractor, in which case the license plate shall be displayed in
  accordance with Section 4850.5.</p>
</blockquote>

<p>So if you were issued one, you can drive with one, and if the department (arguably and <a href="https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=VEH&amp;sectionNum=290." rel="nofollow noreferrer">statutorily</a> "The Department of Motor Vehicles") issued two, you must display both. 
You might think that you could therefore remove one out of state plate when in California, but they also require (as of this year) that</p>

<blockquote>
  <p>(a) A license plate issued by this state or any other jurisdiction
  within or without the United States shall be attached upon receipt and
  remain attached during the period of its validity to the vehicle for
  which it is issued while being operated within this state or during
  the time the vehicle is being held for sale in this state, or until
  the time that a vehicle with special or identification plates is no
  longer entitled to those plates; and a person shall not operate, and
  an owner shall not knowingly permit to be operated, upon any highway,
  a vehicle unless the license plate is so attached.</p>
</blockquote>

<p>The <a href="https://www.oregonlaws.org/ors/803.540" rel="nofollow noreferrer">law in Oregon</a> says</p>

<blockquote>
  <p>A person commits the offense of failure to display registration plates
  if the person operates, on the highways of this state, any vehicle or
  camper that has been assigned registration plates <em>by this state</em> and
  the registration plates assigned to the vehicle or camper are
  displayed in a manner that violates any of the following: (a) The
  plate must be displayed on the rear of the vehicle, if only one plate
  is required. (b) Plates must be displayed on the front and rear of the
  vehicle if two plates are required.</p>
</blockquote>

<p>Somehow, they failed to cover the condition that a person does not have plates issued by Oregon. The <a href="http://codes.ohio.gov/orc/4503.21v1" rel="nofollow noreferrer">law in Ohio</a> is that </p>

<blockquote>
  <p>(1) No person who is the owner or operator of a motor vehicle shall
  fail to display in plain view on the front and rear of the motor
  vehicle a license plate that bears the distinctive number and
  registration mark assigned to the motor vehicle by the director of
  public safety, including any county identification sticker and any
  validation sticker issued under sections 4503.19 and 4503.191 of the
  Revised Code</p>
</blockquote>

<p>This doesn't even make an exception for vehicles from some adjacent state which have only one plate. If you otherwise keep your nose clean, the law says</p>

<blockquote>
  <p>A law enforcement officer shall only issue a ticket, citation, or
  summons, or cause the arrest or commence a prosecution, for the
  failure to display a license plate in plain view on the front of a
  parked motor vehicle if the officer first determines that another
  offense has occurred and either places the operator or vehicle owner
  under arrest or issues a ticket, citation, or summons to the operator
  or vehicle owner for the other offense.</p>
</blockquote>

<p>but you can still get busted for driving your car. Finally, <a href="https://leg.mt.gov/bills/mca/61/3/61-3-301.htm" rel="nofollow noreferrer">Montana says</a></p>

<blockquote>
  <p>a person may not operate a motor vehicle, trailer, semitrailer, pole
  trailer, or travel trailer upon the public highways of Montana unless
  the motor vehicle, trailer, semitrailer, pole trailer, or travel
  trailer is properly registered and has the proper license plates
  conspicuously displayed... (1-plate exception for motorcycles
  etc)...All other motor vehicles must have one license plate displayed
  on the front and one license plate displayed on the rear of the motor
  vehicle.</p>
</blockquote>

<p>One thing to notice is that these laws are in sections on vehicle registration and not operation, where separate sections of the code regarding <a href="https://www.oregonlaws.org/ors/815.030" rel="nofollow noreferrer">street-legal vehicles</a>
prohibit driving without lights or a muffler. Under the full faith and credit clause, I think your act of registering a vehicle in PA is given credit in OH, because the requirement pertains to registering, not the conditions for legal operation on the road.</p>
