Zubats give 1 EV point to Speed..
Kill/Defeat 4 Zubats...that's 4 EVs...when you level up you'll see +1...on SPD..
But if you Kill/Defeat 8 Zubats, regardless of you level, when you level up you'll see +2 on SPD..
I think that's how it works...
But since there's only 255 per stat(can't be divided by four) the rest are scrap..
You get higher pluses since it takes a longer time to kill zubats to level up..so you rack up EV points..and get higher pluses...
Let's have an example..
You're level 20...you haven't killed a single SPEED EV giver...
You kill 8 zubats...you level up..you see +2 on speed..
then
You're level 40...you haven't killed a SPEED EV giver(in this example, Zubat)..
You need to kill about 40 zubats in order to level up..you gain..EXP then level up..you see +10 there..
That's about it..but then there's PKRS..which doubles the EV points pokemon give..
So it only take 20 zubats..in order to get +10..because you have PKRS...
signed-
Gregory
If anything is wrong, shout out.