2004 Bonneville speedometer is off about 5 mph. What do I look for?

Check tire size for proper size. The speedometer on the Bonneville is strickly electronic, calculated by pulses from vehicle speed sensor through ECM. ECM sends signal to instrument cluster which converts to MPH or Kilometers. If the car has had overisized tires on at one time it is possible to reconfigure the ECM for non OE tiresizes.