They hired him to be a driver, but Rex found that he did very little driving nowadays. Possibly because his employers thought his skills as a mechanic would be more valuable to the company – he did, after all, know basically all there was to know about the cars in this era – or it might have been a complaint lodged against him by one of the bigshots he was ferrying around, for 'reckless driving'.
Finesse, recklessness; tomayto, tomahto.
At least this meant that he didn't have to wear suits (you'd think a man who used to run around in a leather outfit would be okay with a three-piecer, but apparently not), and he could get out once in a while. For Rex, hanging around services stations and talking to people about engines and oil changes was like a party: he was now on a first-name basis with the attendants at the station near Stark HQ, because well, he was him.
The topic of the day seemed to be octane ratings, and it was getting a bit heated. "150 octane in a car? What kinda super truck would you have to drive, Rex?" A redheaded young mechanic by the name of Aaron was arguing. "Imagine the supercharger in that thing! It'll be bigger than a fighter jet."
Rex shrugged, half-laughing. "All I'm saying is that it's possible. In the future."
This was why he didn't really take the 9 out any more. The gas quality in the 60s was shockingly bad, and before he made some modifications, he didn't want to put any of it into his car.