speed-o-meter is wacky +5???

Double check your number. If it's tires or gears your speedo will be off by a percentage as opposed to a set MPH. So if it's 5 MPH off at 30 it will be 10 off at 60, 15 off at 90 etc... If in fact it is a set MPH off at any and all speeds I would look at the unit itself.