It's not impossible. In fact, it's the only way it can be. If a car is locked in a gear, its wheels (mph) are mechanically linked to the engine (rpm). They vary together linearly until the torque converter gets in the game (or your foot with the clutch). If someones sees a non-linear dependence between car speed and engine rpm _while in the same gear_ it's because of a torque converter slipping (if the torque converter is locked and this still happens, then you have a _faulty_ torque converter). I repeat: once locked in a given gear, your speed will _always_ vary linearly with engine rpm. That's what a gearbox does.