Not necessarily. If the speedometer error is proportional to speed then the odometer would also be off. For example in your case the error would be 18mph at 100mph and 4.5 mph at 25mph. If the error is around 9mph at most speeds then the odomoter is not necessarily wrong. Think of it as the speedometer face being rotated a bit.
In reality you'll probably see a bit of each, where the error increases a bit with speed. My E24 and E9 are off by about 4mph at most speeds.
The main question is if the speedo has recently become flaky or you always had this discrepancy. Wheel sizes and differential ratios will affect the speedometer and odometer with an error proportional to speed.
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