does the tach read right when u get a aftermarket final drive. i know at high speeds the stock one isnt that acurate but does it get worse or is it the same

 

You meant speedometer....

To simply answer your question, with the final gear changed, your speedo will be off. It will read higher than the real speed, for ex: in real it is 55 but on speedo it will read 60.
This also translate to higher reading in odometer.

But you can countermeasure that by installing electronic device aka 'yellow box' to correct the error.