Lowest most efficient low load rpm w/ 350

[original condor]

I'm in no hurry usually so I run @ 60 most times, lots on 2 lane roads, plenty of time & shortage of $$$.
I've got 308's and 33"ers, with a mild build 350 & SM465, rolling on flat ground most times. This calcs out to approx 1900rpm, with 35's it drops to 1774. Is this too low, or will it get less thirsty down there?
I'm considering 35's for longer freeway legs if in a hurry, and more clearance under the diffs.......Thankyou...... Steve Rieck