There are two sets of regulations IIRC. If it’s older than 25 years on import, you can bypass FMVSS/EPA/DOT regulations, so they are always perfectly legal to

There are two sets of regulations IIRC. If it’s older than 25 years on import, you can bypass FMVSS/EPA/DOT regulations, so they are always perfectly legal to drive on the federal highway system (incl. at 70-80mph). Some states will refuse to register them though, such as Minnesota or Oregon. Separately, some states have an import process for cars newer than 25 years old which allows them to be licensed as UTVs/ATVs, so they can go on local roads but are usually speed limited. And iirc, they can’t drive on federal highways.