Former Tennessee Titans offensive coach Todd Downing has turned himself in ... and will begin serving his 48-hour sentence stemming from his DUI arrest in November.
TMZ Sports has confirmed ... Downing surrendered to the Williamson County Sheriff's Department Tuesday and is in police custody at the Williamson County jail.
Downing was arrested in November ... just hours after the Titans defeated the Green Bay Packers on Thursday Night Football.
Tennessee Highway Patrol officers claimed Todd was "traveling at a high rate of speed" on Interstate 65 at around 2:30 AM.
Downing, 42, was arrested and booked on DUI and speeding charges.