How to play this "Down The Road" Riff

Hi guys, I have having lots of trouble playing this riff without having the speed, I can't seem to figure out if I am picking with my middle/ring/or thumb on this lick?

---(T)--(I)-------(I)-------(T)--(T)--(M)

But this makes me have to strick the thumb pick very fast on 2 strings towards the end, I am sure yall have a better idea of how I should be picking this to be more efficient?

Thanks!