As long as you don't have wheelspin when passing 100 km/h, the OBD2-speed should be rather accurate. I used that with RaceChrono to measure the 0-100 time, and beat the factory figures by one tenth of a second.
The more power you have, the more difficult it is to get a good run with manual gear. It depends on what tyres you have. With winter tyres, you need more wheelspin.
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