[niky]

Power = F * V = MAV?

But the engine isn't putting out power. The engine's ability to cause acceleration is determined by the force at the wheels. Not power at the wheels. (Try looking for a chart... any chart... that shows horsepower at the wheels in different gears).

If force at the wheels is constant, then acceleration is constant, or as constant as can be dependent upon slippage, drag, etcetra. Constant application of power equals constant acceleration

Acceleration is directly linked to wheel torque, which gets lower at higher speeds because of lower gear multiplication at higher gears, which makes more acceleration at the engine result in less and less acceleration at the wheels. Not because of the speed of the vehicle itself.