I just last month switched out my rear end that is behind my T5, I had a 3.0 and switched to a 4.11.( 3.0? don’t ask, came to me w/ a Currie rear housing).
I would suggest focus also be on the overdrive ratio/cruising speed in addition to 1st gear.
My experience in my 46 1/2 ton was that with the 3.0 rear gear is that for 5 years I never ever used 5th gear, my truck would highway cruise perfect in 4th, about as fast as I ever want to go in a truck this old, even though the suspension/steering is 100% redone and aligned, and newer bendix brakes and master fitted and a fresh full pressure 235. I felt that aerodynamics kept 5th gear for only if I were to drive off a cliff.
On the first gear side yes it is slightly low (and I now shift at the other side of the intersection to 2nd) but it is noticeably easier on the clutch and as important reverse is now a lower ratio making backing into the tight garage a whole lot more controllable.
I used one of the many online rear ratio calculators and input rpm, tire height(29.3” for mine), trans gear ratio, and rear ratio to show the mph.
I’m glad I went thru that process, thought I wanted a 3.90 but for mine the numbers didn’t say that, I am pleased with the result, and I can use 5th on the highway now…
chuck
