Some information I found from DMV:
www.dmv.ca.gov
"To be eligible for on-highway registration, your OHV must be manufactured for both on and off-highway use.
Motorcycles manufactured after 1978 with an engine size of 50 cc or larger must have a label indicating that the vehicle meets CARB emission standards and a manufacturer’s certification for on-highway use.
ATVs do not meet federal standards for on-highway use and cannot be converted.
If your OHV has a red ID plate, it cannot be registered for on-highway use.
Exceptions:
Off-Highway Vehicle Registration - California DMV
Off-Highway Vehicles (OHVs) are used only on lands that are open and accessible to the public. OHVs must display an ID plate or placard issued by the DMV.

"To be eligible for on-highway registration, your OHV must be manufactured for both on and off-highway use.
Motorcycles manufactured after 1978 with an engine size of 50 cc or larger must have a label indicating that the vehicle meets CARB emission standards and a manufacturer’s certification for on-highway use.
ATVs do not meet federal standards for on-highway use and cannot be converted.
If your OHV has a red ID plate, it cannot be registered for on-highway use.
Exceptions:
- Motorcycles manufactured before 2005 with an engine size of less than 50 cc are exempted from emission label requirements.
- Motorcycles manufactured after 2006 with an engine size of less than 50 cc may have a label indicating that they meet U.S. Environmental Protection Agency (EPA) standards for on-highway use in lieu of a CARB label."
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