One of the reasons I love my GC8 JDM cable drive setup It's always spot on no matter what ratios I run.

You can get kits (Jaycar) for adjusting speedos (would I use one, dunno), or you can get a good gear calculation program from the web, punch in your ratios and drive by tacho (the tach will always be right) or you could consult the yellow pages for instrument service companies that fix speedos.