The rumor is that you can hook it up going straight to the speed sensor and you dont have to go throgh the computer. Right now while going through the computer if I go 5mph it says 40mph.

Here is the web site I found it at

http://www.autometer.com/hp/index.html

then click on the faq and its in automotive eltronic programable speedo

 

There is also a company, www.dakotadigital.com , that makes a device to correct your speedo. I think theirs is about $90. The nice thing about it is that you can change it later if you put in different gears and such. Good Luck.