Quote:
ewi said:
Quote:
69bossnine said:
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A Rennteamer with new tires would show slower speeds at a given RPM than a Rennteamer with bald tires.
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Shouldn't it be the other way round?
speed = wheel RPM * wheel circumference
and wheel RPM is mechanically fixed to the engine RPM in each gear.
So, if RPM is the same for both cars but wheel circumference is greater for new tires the car with the new tires should achieve higher actual speeds. Don't forget to break in the new tires if you try that yourself

The displayed speed is another thing and depends on the circumference that is used in the software. Assuming that they use a fixed circumference of a new tire and not some characteristic curve dependent on tire wear (would be interesting to know how they would try to calculate that) and speed itself (wheel circumference increases with speed) the displayed speed does not depend on tire wear. In that case the displayed speed of the two cars would be the same.

997S: 300km/h @ 7200RPM, obviously in 6th gear



Theoretically speaking you're absolutely right, although the equation v=w*R (w stands for the rotational speed) is just a simplified algebraic expression of the circular motion and NOT the algorithmic model used by nowdays manufacturers.

Besides, the principle of the car speedometer as described doesn't actually incubate the modern forms of digital devices that have NOTHING to do with the wheel diameter just becuase they aren't directly linked with it.

In fact current speedos employ a rotation sensor, to be found on one of the driveshafts -not on any of the wheels. By definition, that sensor generates a series of electronic pulses whose frequency corresponds to the rotational speed of the driveshaft. The signal is then transmitted to the ECM that converts the repetitive pulses of any frequency to a meaningful speed value before being displayed either on an electronically-controlled needle reading or a digital output display.

Hope this brings our understanding closer to modern cars reality.