You cant zero a stat sorry mate. There is a difference in speed and fitness. If there are a group of zombies and you have speed 3, you'll be able to run through them and dodge with only a small chance of getting grabbed. If you have 2 you have to be very careful because the shamblers are the same speed as you. Fitness is exactly how it sounds, it covers your fitness. With 3 you can jog a fair bit and fight close combat for a good ten minutes. With 2 you can probably jog for the length of one street before getting tired or only fight close combat for a few minutes.