That's a known issue with most Highlander drivers I've talked to. You might like to read through this thread and also this thread that deal the speedometer error issue. Hope that helps.My 08 HL speedo seems to register about 2 mph faster than my actual speed. My GPS always reads 2 mph lower as did three different "You are going XX mph" speed detector. Anyone else experience this? I have the limited awd if that makes a difference.
Also, it depend on which wheels and tires you have mounted too. Mine is the Base trim with stock wheels and tires, and my speedometer registers 3 MPH too fast.