From my experience, this happens quite often in Honda Accords. It doesn't occur in CR-V's or Civics. I have used my GPS to watch my speedo in all scenarios. Civic's (03 Hybrid and 99 hatch) matches the GPS; so does a 06 CRV EX.

My brother's 02 4 cyc accord EX and my 02 v6 EX are about 4 mph slower than the GPS. Off course, these are the standard stock size with correct tire size. Don't know, it may be an engineering problem with the cluster. This appears to only be happening in the 5th and 6th gen accords. I don't recall my 91 Ex doing this.