why is my lexus ls400 speedometer 01 to 15 mph off

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We have a 1996 Lexus LS 400 and the speedometer reads 10 to 15mph off on the low side. Example: Speedometer indicates 15mph, actual speed about 25 mph. 35mph indicated, actual speed 50mph. What needs to be replaced to correct this issue. Thank you for your answer. GB