Why is Engine RPM not directly proportional to Vehicle Speed, even when gears are not changed?
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This can only happen in normal operation when you’ve got an automatic transmission.
There’s a component in an automatic transmission called a “torque converter”. This is a fluid coupling…a “propeller” and a turbine right next to each other with oil in between. It provides two important functions…when the car is stopped, it lets the engine keep turning without needing a clutch to disconnect the engine from the wheels (the coupling just slips). And it allows some mismatch between the engine speed and transmision speed…through some messy fluid mechanics (which we can get into if you want) it acts like a reduction gear, allowing the wheels to spin more slowly and increasing the torque.
This can happen in manual or DSG transmissions if the clutch slips, but then it’s a bad thing.
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