Depends on your speedo. The early 140 odo was mechanical and as susepible as all speedos to manipulation. All replacements are digital and they are alterable right through the diagnostic port. They are now the least protected they have ever been. See this site: http://www.megachip.de/en/index.html
There are numerous software folks doing this type of repairs. The manuafacturers are trying many different hardware and software locks but this stuff ain't national security and they have trouble keeping that under wraps.