[sammyg2]

I've done it for several reasons, on the 911 speedos it was for stripped odometer gears.
If yur speedo is actually 6 mph high at any speed, moving the needle 6 mph lower would solve the problem.
Usually the mis-calibration is progressive, or a percentage. It might be off 1 mph at 10 and 2 off at 20 mph etc.

The hardest part is peeling back the bezel metal ring thingy (technical name) without trashing it. That, and getting the needle off :0

I believe there's a technical article on this sight about speedo repair.