As much as Tony Stark was a non-entity back home (way before his time, Rex supposed), he was witnessing first hand the kind of magic Tony could do. Advanced science was the one that was indistinguishable from magic, right?
"I can do one better." After all, he was a mechanic, not an engineer, and Rex wasn't sure he'd be able to remember all the things he read go into workable, suitable gasoline. There was a small plastic container, discarded haphazardly under the passenger's seat, which he retrieved and shook lightly, revealing that it was half-filled with liquid, and should be bug enough of a sample.
Handing it over to Tony, he explained, "I siphoned the rest out when I was installing the filters."