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BMWs are off by the most, reporting 73.18 when doing "true" 70 mph. European laws are more strict than US, requiring speedometers never to underreport speed, even with oversized tires. Their theory is that since people are more likely to put oversized tires on BMWs and Porsches, they have to have the most overhead.
Beyond that, the article lists various reasons for speedometer error. Riez's tests compare speedometer and tachometer readings. The speedometer is definitely not a direct measurement. I am not sure what errors might impact the tachometer. There could just be numerous small errors that impact 2 indirect measurements that result in the disparity with Shipo's theorectical results.
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