Update:
By default CF uses UTF-8 to decode URL variables. "To decode .. URL variables in any other encoding ... you must use the setEncoding function."
<cfscript>
setEncoding("URL", "shift_jis");
writeDump(URL.brand); // result 日�Y自動車
</cfscript>
When you have questions like this, the first place to look is in the Functions by Category view of the documentation. All of the functions are descriptively named. You can usually figure out the function you need by its name alone. If you look over the String Functions section, you will find two functions that (combined) should do the trick:
CharsetDecode(string, encoding)
- Decodes a string in the given encoding into binaryCharsetEncode(binary, encoding)
- Converts binary into a string using the specified encoding