Light weight rear gear option? RPM/Speed calculator?

I've found some of these calculators online but dont have any info in front of me that I need, tire height? revolutions? Clicked on Eatons and it tries to open some some file my pc doesnt recognize.

Current setup:
01 KW T800 Detroit 12.7 430/500hp 1650 lbs/tq ....Trans: RTLO16913AP......Rear - 3.90's Tires- 11r22.5
Weight of the truck and load combined will never be over 30k lbs. Region - Everywhere in the continental states OTR. No one place more than the other really.

I am not driving the truck now, working on it, but I think 70mph was around 1600rpms in 13th gear.
I'd like to do that in 11th (direct) or maybe even drop it to 1500rpms @ 70mph.

I was thinking 3.50's but cant find a chart/calculator to see where it gets me. After reading some posts on here 3.36 might be better? 3.08's ?

Anyone have a chart? RPM/speed/rear gear, or a Calculator?

 

Thanks. That equation will work for direct gear 11th 1:1 but when you go into overdrive the input to the rear end no longer matches the engine RPM. I dont think each gear is a full 400rpm? I dont want to guess. Im assuming if 1000rpms of the driveshaft would be 860rpms at the engine in 12th, and 730rpms in 13th. I was hoping for a calculator or chart so I dont have to figure out all the math myself. lol. I would think there is a chart somewhere that tells you the RPM at a given speed, and then the variations based on different rear gears.

13th - .73
12th - .86
11th - 1.00

 

It tries to open a .swf file and im not sure what that is so it doesnt work.

 

I measure the 11r22.5 tires i have off truck and get 40" tall. On a few different ones. That totally contradicts any info on here that they should be 42" tall and LPs are 40" tall. So that kinda throws out any calculations i can do because not sure what tire height is.

 

I just used 41" tall, the variance should be small enough. I am making a chart with all gear ratios, rpm, and mph. taking forever but should do the trick.