Re: Level at which it would be clear what spd iv growlithe h
A level 99 Arcanine has 222 speed for an IV of 30, 223 speed for an IV of 31 and 224 speed for an IV of 32, so there are no 'forgiving' coincidences in this case, every increase counts.
Your growlithe is actually 30-32 at the moment (veekun does not output the 32 IV possibility, but my calculator does: http://forum.pokemon-world-online.net/showthread.php?tid=8417). The earliest following levels at which you can distinguish these values from each other are:
a) At level 35 you will distinguish an IV of 32 (=58 speed) from 30-31 (=57 speed)
b) If you note speed=57 at level 35, you must wait until level 51 to distinguish between an IV of 31 (=82 speed) and an IV of 30 (=81 speed)
Hope that fully answers your question
Cheers
P.S. If you wanna do lots of similar calculations, check the link I have supplied above. My calculation tool is much more convenient than veekun for these things.