Any "on-road" motorized vehicle (less than 25 years in age) must meet US safety and environmental standards to be imported into the United States. A compliance letter must be obtained from the actual manufacturer (not dealer) stating that the vehicle does meet all FMVSS standards. The compliance letter is not required if it is a return of a U.S. registered vehicle. A list of automobile manufacturers can be found on the National Highway Traffic Safety Association – US Department of Transport's web site (link is below) in the section called "Vehicle Importation Guidelines- Canadian".
You should be aware that some manufacturers are refusing to issue the compliance letters (i.e. Chrysler, Toyota). As well, not all motorcycles can be imported to the US, particularly those that were manufactured overseas or are considered gray market bikes. If you cannot obtain a compliance letter, then you must use the services of a "Registered Importer" to determine if your car or motorcycle meets or can be modified to meet US safety and emission standards. A list of registered importers is also located on the NHTSA website (link below).
An EPA (Environmental Protection Agency) form must be completed for each motor vehicle being imported into the U.S and is required in addition to the compliance letter. The EPA form is not required if it is a return of a U.S. registered vehicle.
A HS7 document is required on all motor vehicles, including a return of a US registered vehicle. This form can be found on the NHTSA web site along with full details on vehicle importation:
If you will be in the US for one year or less, you can import a non-conforming vehicle, however the vehicle must be exported within one year – no extensions are permitted. It may be necessary to post a bond for this type of importation.
Returning US citizens/residents who have purchased a vehicle while living in Canada should be aware that duty will be charged on any automobile that was manufactured outside of the NAFTA zone. Duty is currently charged at the rate of 2.5% (subject to change).