I have ran up to an 80 in season. I use this guide
https://www.thebuddyforum.com/demon...238762-2-4-dmo-db-wizard-guide-70-grifts.html
Here is some data on 75's I ran today while at work.
[Adventurer][Rift] Runs count: 32
[Adventurer][Rift] This run time: 0:10:56
[Adventurer][Rift] Average run time: 0:14:09
[Adventurer][Rift] Best run time: 0:10:37
[Adventurer][Rift] Worse run time: 0:18:35
[Adventurer][Rift] This run XP Gained: 21,435,036,197
[Adventurer][Rift] This run / Hour: 117,539,189,001
[Adventurer][Rift] Total XP Gained: 699,887,017,404
[Adventurer][Rift] Total XP / Hour: 92,680,257,763
[Adventurer][Rift] Best XP / Hour (single run): 131,729,612,293
[Adventurer][Rift] Worse XP / Hour (single run): 69,234,326,329
[Adventurer][Rift] Best XP / Single run: 24,486,466,533
[Adventurer][Rift] Worse XP / Single run: 18,611,206,637
here is a d3planner of my gear to compare
http://www.d3planner.com/369660824
If you have mostly ancient with decent rolls, augmentations of the legendary gems on your gear, then you should be able to run 70+ pretty easy. Fair warning, do not expect to be able to 100% complete everytime anything above a 75-77 because the grifts can make or break you pretty easy.