Rear end help, 390 or 370 hauling cattle.

Then you might want to run taller gears to lower your rpm. Taller as in like 3.22 or lower

 

My 379 with a b model cat and a 14715 with 4 and 5 gear flipped backwards running thru 3.70 ratio on tall 24.5 rubber would run 75mph at about 1700 rpms 80 mph at 1800 that transmission overdrive gear was .079

 

I have a 90 model 379. 15 over 3.36 rears tall tires. 80 is 1600.

 

This has me thinking hard?
My old truck had lo pro 22, was 3.55 axles, with a 10 speed OD, and I ran 75mph or 80 mph at 1750ish. .73 overdrive. Would max out at 96mph 2100 rpm.
Hauling reefer.

 

There it is black and white at 1400 and 1750. It's a 15715 with .79 overdrive. Tall rubber and a B model CAT. 11R24.5 rubber. It's a triple digit truck loaded or empty. Nothing to wonder about

 

Series 60 12.7 is the main difference.

 

No it isn't
RPM through a set of gears is the same regardless of engine. Tall tires and higher rears is the difference

 

Yes. I was a bit confused.

 

I ran the calculator on my project W9 and it "should" just hit 100 mph at redline with 3.73's and the same transmission ratio and tire size you're running. Not that I'd ever think of trying that. 70 is plenty fast lol.