> Speed limits exist to reduce fatalities

In the USA the national speed limit was implemented by Richard Nixon in response to the oil crisis of 1973. The motive was to reduce energy consumption, not to reduce fatalities.

https://en.wikipedia.org/wiki/National_Maximum_Speed_Law

As you might note in that link, the states already had their own speed limit laws in place, and that the national limit was later repealed. Thus, the speed limit laws are not in place to reduce gas usage.