Using a timid Altaria, what significant threats are outsped by adding how many Evs?
going with 252, it's base speed at lvl 50 reaches 145, but this seems unnecessary.
Can I get away with adding less, perhaps just enough to have it pass 120? (172 I think?)
So how many EVs would be worth investing depending on what I wanted to go faster than?
IS there a better way to word this question? Is my actual question even clear?
I will give you a list of speed tiers you can outspeed with what investment, since a lot of pokemon under 80 base speed tend not to run max speed.
Outrunning base 60's with a neutral nature and max speed will require 20 speed EV's.
Outrunning base 60's with a positive nature and max speed will require 100 speed Ev's
Outrunning base 65's with a neutral nature and max speed will require 60 speed Ev's
Outrunning base 65's with a neutral nature and max speed will require 140 speed Ev's
Outrunning base 70's with a positive nature and 252 speed EVs will require 20 speed EV's.
Outrunning neutral base 75s with max investment requires 132 speed Ev's
Outrunning positive speed nature base 75's, with max investment, requires 220 speed Ev's
Personally, I'd either run 220 or max it out. A few mons lurk in between base 75 and 80 them being: Vanilluxe (79); Blastoise, Feraligatr, Diggersby (78); Heatran (77); and Nidoqueen and Hitmonchan (77), so you can plan accordingly if you wish to outspeed those.
I hope I helped a little bit, and I apologise if this wasn't as helpful as you had hoped for.