In the end they are set by politicians. Not for malicious reasons; politicians don't need to be malicious in order to make bad decisions, they have plenty of other reasons for that. In many parts of the US, surface road speed limits are quite reasonable, but freeway speed limits are ridiculously low.
You are correct, by the way, as far as I know, that the 55mph speed limit was originally an energy conservation thing. This was during the oil crisis of the 70s, when it was federally imposed. Ever since the federal mandate was lifted, speed limits have been creeping back up, but at very different rates in different states.
To give a comparison, in France freeway speed limits are roughly 80mph in dry weather and 70mph in rain (I say roughly because they are, of course, in metric.) In Belgium, and the Netherlands it's 75mph, although The Netherlands has introduced many variable speed limits (electronic signs based on congestion) and recently bumped it up to 80mph on certain rural stretches (rural by Dutch standards). Germany, of course, has no speed limits at all on many long-distance Autobahns (about 50% of the network), although in metropolitan areas, contrary to popular mythology in the US, they often do have speed limits, which go by the charmingly long-winded name of "Geschwindigkeitsbeschränkung," often shortened in colloquial speech to "Tempolimit." The de facto speed limit on the unrestricted Autobahns is 125mph, since that's the fastest unmodified German cars will go.
So did some technocratic bunch of engineers evaluate the conditions in each of these countries and decide that somehow some subtle difference of geography that Germans are capable of safely driving a full 65 mph faster than Hawaiians?
I doubt it.
May I also remind the reader that East Germany used to have a rigorously enforced 60mph limit that was rather promptly lifted after the reunification, which was by no means an event of particular relevance to traffic engineering.