The chuggishness, classicism versus bluesiness, and rhythmic undulations are generally what set them apart. Speed metal is best defined as what Motörhead invented with the title track to Overkill. Fast paced, breakneck, no breakdown, usually blues-inflected, borderline if not completely chugless, extreme metal with a usually at least slightly gritty vocalist. Thrash is more prone to chuggier rhythms, breakdowns, often favors classicism to blues influence, and has a bit more "unka chunk" to its beat so to speak. Thrash also has the ability to be a lot more techy, whereas speed is generally relegated to a punk rock esque simplification reflective of its creation by Motörhead. Not to say it's too simple, it's still got the generally higher technicality characteristic of most metal, but it never gets proggy or even as high brow as some of Megadeth's more challenging stuff.
