Technical Overdrives, engine size and rear gear ratio- thinking out loud

Most foreign cars I've owned have had a 4.10 "rear" or so. My '12 Malibu has 2 overdrives I believe with the 6-speed automatic. Revs are way down even at 70 or so. You need to plug and chug all the possibilities and I like the Wallace calculator myself. I run a 4-speed with a 3.89 rear and even with the OG 3.56 gear I was grabbing for the next gear. I may pop a 5-speed in it one day. At 70 I'm turning 3250 RPMs.

 

Just put an OD behind my 230. Still has its 4.11 rear. Used to drive indicated 65 on the highway 62 by gps. With my 28.3 inch tires it was turning 3280 .With the OD it's 2300. The fellow who did my machine work said a safe cruising rpm was 80 to 85 % of where the factory pulled max hp. Mine is 3600. The 2300 in od is still in the usable torque band and it handles interstate grades with ease. Mileage went up by three or four mpg in the bargain. With the 4.11's is still peppy in traffic or on the two lanes. My engine is modified makes 127 hp at the rear wheels at 3200 on a chassis dyno. If your engine is low hp, you want to keep the rears low for around town.