Did anyone actually slow down to 55 MPH in 1974?

Some states went to alternate day fuel purchases based on the tag number.

Yeah, California did that. Except commerical-plated vehicles were exempt from the rule.
At the time we owned a '74 TransAm and a '69 Ford F250 with dual tanks. All pickups in California have "commercial" plates so I could get gas anytime. I used to keep one tank full and would refuel the TransAm as necessary from the 2nd truck tank if it wasn't the right day according to the cars plate.