Its called engine braking. Unless you put it in neutral, when you let off the gas, the car will decelerate as the throttle plate is closed, the RPMs drop, and the car is still in gear. This is normal no matter what car you drive.
If you put the car in neutral and let off the gas, then you should be able to coast for a pretty good distance without slowing down too much. But if its in gear, then the trans is still engaged and the friction of everything(wheels, trans, moto, weight of the car) will cause the car to decelerate.
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