the multimeter shows less reading when car in on rather than its off!

What is your question?

I have changed my battery, and the multi-meter shows 12.57 when the car is off. But, when I start the engine the multi-meter shows 12.25!! which it should be at least 13.7 I guess. Could you please tell me that why the reading when the car is on is not even equal to what it is in off