C being? Or are you describing the change of velocity over time, which is delta-V over T? Which is acceleration?
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The acceleration tapers off (in the absence of drivetrain losses / frictional losses / air resistance and whatnot) not because of veloctiy, but because the time required to apply the force increases due to the lowering gear ratio. If the force could be applied directly to acceleration without having to match the speed of the road, then there's no such problem. It's in relation to the wheel speed that we start to lose acceleration. As wheel speed increases, we apply less and less force over time.
Remember: Power in relation the acceleration of the vehicle itself describes what is happening because of the application of Force. It is NOT what the engine is putting out.
Some further reading:
http://forums.xkcd.com/viewtopic.php?f=18&t=42952
http://www.daviddarling.info/encyclopedia/T/thrust.html