You don't.
Don't get me wrong, but even with those psuedo-legendary stats, Goodra is just not built for this. That is to say, as a member of the Mighty Dragons (lol).
The only way to substantially increase speed is by a Choice Scarf, but that isn't too good as Goodra is stuck with using only one move, which is an okay-sh situation, but you're better off with a Leftovers or a Life Orb.
Then again, even if you do choose a Scarf, it cannot out-speed most of the Pokemon that it needs to, and it would be a waste anyway.
I agree 80 base stat in speed is quite good, but with its move pool, I suggest trying other roles, i.e. that which isn't a fast sweeper for your Goodra.
You can still stick an Assault Vest on it though, and try to be a Tank of sorts.
Alternatives, as Astro said, is to lower your opponent's speed, either by paralysis or Sticky Web
tl;dr: Don't try to increase its speed. If you do, use a Choice Scarf, but be careful.
