That would be I-10 with a posted limit of 80mph (especially going west from San Antonio). It was 80 the last time I was down there about a year ago. There are also stretches of highway in West Texas with posted limits of 80mph.
I live 2 blocks from I10 in Houston and I have driven it all the way out to west Texas. I don’t recall any 80MPH speed limits, but if you say there is, I’ll just have to take your word for it.