Nixon left office in August of 2004. It's my understanding that the 70mph speed limit was still in effect.
Richard Nixon
signed the Emergency Highway Energy Conservation Act into law on January 2, 1974. It essentially set the speed limit at 55 MPH by denying federal highway funds to states that didn't lower their speed limits to 55, although it may have taken some time before some states enacted legislation creating a 55 MPH speed limit. Nixon left office on August 9, 1974.
The 55MPH speed limit is arbitrarily low for most suburban and rural interstates (and many urban interstates outside of rush hour), and is
lower than the speed limits on most motorways in more energy conscious Europe and Canada. I see little reason to go back to it. Time is money.