There are various factors that must be met for you to earn an A-Rank. The big one is how long it took you to complete the game and various smaller factors as well. Each requirement earns you so many "points." To earn an A-Rank you must meet a certain number of points at the end of the game.

Time Score
04:30:00 or below 8250 points
04:30:01 - 05:00:00 7550 points
05:00:01 - 05:30:00 7000 points
05:30:01 - 06:00:00 6450 points
06:00:01 - 07:30:00 5500 points
07:30:01 - 15:00:00 5000 points
15:00:01 - 20:00:00 2500 points
> 20:00:00 2000 points
*The clock is ALWAYS running, there is no way to stop it.

Special Score
1) Did you give the Hemostatic Bottle to the prison guard?
Yes = 250
No = -1000

2) The speed it took you to save Steve from the Gold Luger Trap.
Fastest = 400
Average = 0
Slow = -1000
*There is no known numeric value, clearing the 4 zombies from the hall outside the room helps with this though.

3) Map discovery
100% = 100
Anything less = 0

4) First aid spray
No use = 1800
If used = 0

5) Typewriter saves
No save = 0
First time = -1000
Every time after = -50
*The free save after killing the Tyrant and before landing in Antarctica does not count.

6) Retry
No Retry = 0
First time = -1000
Every time after = -50

Total Score
10000-10800 = A rank
7000-9950 = B rank
4500-6950 = C rank
2500-4450 = D rank
2450 or below = E rank

As you can see, there are many things you must do to be sure you make it.