GER was the worst converter company on the face of the earth.
I've always used this RPM calculator, it's very simple and accurate.
https://www.ringpinion.com/Calculators/Calc_RPM.aspx
Original poster, what motor are you running?
I've read alot of threads about highway gearing and have one of my own and for the most part they're just a pee'ing match since it's all about personal preference. Some guys are fine with roaring down the highway at 3800 RPM's. I am not ok with that as it just simply annoys me.
My Duster has a 500" big block with a 727, 9.5" Dynamic converter, and an 8.75 with 3.55 gears along with rear tires that are 27.5" tall. I'm at 2800 RPM's at 65mph, yesterday I installed 3.23 gears which should bring it down to around 2400 RPM's.
When I have the energy, hopefully soon, I want to pull the 9.5" Dynamic and sell it or trade it for a tighter 10" converter. I drive my car alot and the motor has so much torque that the 3.55 gears feel like 4.10's so I'm thinking I'm gonna like the 3.23's.