Car batteries are designed to drain and charge partially, but rapidly. Unlike most batteries that are deep cycle, they can drain fully and NOT suffer any deterioration. A car battery, you drain it more than halfway, sure it charges back up fine, but its taxing on the life of the battery and it will fail sooner.
If the start demand on the vehicle is NOT that great, the smaller battery may be fine. But, even if the vehicle seems to be starting fine, it may NOT be obvious how taxing it is on the battery.
Regardless, its all academic, if the car is starting fine and don't have to worry about really cold weather, the worst that is going to happen is the battery is NOT going to last as long as if you had gotten the recommended size battery.
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