Assuming the player does not kill T-Ray in the process of recovering the Highwayman, T-Ray can be contracted to perform upgrades on the player's car. Initially the only upgrade available is increased trunk space for $500 ($300 if the player passes a Barter check), however, upon completion of the game's main story line the player may opt to have T-Ray install grav plates to significantly reduce the car's fuel consumption.
If you sleep with him to get your car back, and seduce him again for the upgrade, he will say "Damn, woman! I just gave you sweet love five minutes ago!" This is a reference to Chef, a character from the show South Park, during the episode, 'Weight Gain 4000'.