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hearnia_2k t1_j1ewg7i wrote

Reply to comment by OldTimeyMedicine in Speedometer of a newer Mustang reads "Ground Speed" by egordoniv

Wouldn't it be both? Equal and opposite forces blah blah means if a car drives in one driection then it applies the same force on the ground in opposite direction. So, the ground speed is the different between the speed of the ground in one direction and the car in another direction.

Otherwise the car would have to do some pretty unnecessary calculations to know how much force the car was putting on the earth in the opposite direction, and how much that impacted the speed of the earths rotation etc. Plus the speedo would likely have to show pretty close to 1.3 million MPH?

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OldTimeyMedicine t1_j1ezp6r wrote

Setting aside speedos for a moment, the Car Speed would be relative to the frame of reference, or more appropriately, the rate at which the wheels are turning since the speedometer is measuring that based on the wheel rotation. So if the car is drifting, the speedometer will present the speed according to wheel speed rather than the resolved components of the speed vectors, where the datum isn't defined here.

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hearnia_2k t1_j1ffmaf wrote

Yes, it's true, and I even mentioned this in another comment, but it's also not what you previously states, where you said it's the speed the car is actually moving; but it's not, it's based on having a value for the outer circumference of the tyre, and speed of rotation etc.

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