Spec'ing auto vs manual.

Wondering how to gear a new truck with an auto trans to get the same performance as the manual

Engine is Cummins ISL 370 hp 1250 torque

manual config..which i have been happy with is
3.90 rear
510 rev michelin xda5+
.74 overdrive on eaton 10 speed
this puts my rpm at 75mph at around 1840

dealer proposed with auto
4.33 rear
510 rev michelin's
.65 overdrive in allison auto 3000rds
that puts my rpm at 75mph at 1794

options for rear end include a 4.63, 4.88, 5.29
In another truck i have same setup with a 4.88 , auto. 75 @2022rpm and it has been fine, but a smaller engine 325hp , 750 torque

 

Refrigerated fright
10 wheeler
Mostly highway maxed out and lot of steep grades when off highway

 

It is to a certain point.. in the auto truck with a 4.89 I get 8 mpg, in the manual with a 3.90 I get 6mpg

 

Not at 75mph you won't