I have no idea what transmission is in there other than it is a "Short Shaft", it is the only one I have in a vehicle now with a short shaft, I could likely convert over to a long shaft but it has a custom length driveline I had made, I prefer long shafts because I do not have to lean forward to shift into 5th gear like I do this truck, but that does not stop me from driving it, it is my favorite vehicle to drive.
At 70pmh I show right around 2500rpms, as I recall the last time I had it going 100mph it showed about 3900rpms but I might have been going a little more than a 100mph, this person was bugging me so I left him behind, in my rear-view mirror I seen him take the next exit.
I want better gears because of another reason also, 70mph is 2500rpms, but the engine likes 2300rpms, to get that extra 200rpm I have to step deeper on the pedal, I get 30mpg going 75mph, if I could keep in under 2300rpms I would likely get 35+mpg as I have done that doing 65mph(under 2300rpms), the guy that I bought the other SD25 engine I have in the 720 from drove 50/55mph on the freeway, and with propane injection(crude setup) he claims to have got 48mpg, I have that setup to this day, but I considered it a hassle and did not want to constantly be thinking about it, plus I cannot drive 55mph, I never did try to see what kind of mileage I could get going 75mph because at that time the EGTs on that engine ran way too hot to be driving it that way, this was before I figured it all out and it does not run hot like that anymore but it has a different turbocharger and is not the same as my 521 kingcab, but it will pull anything behind it, it has way more torque than my 521.
By the way, all my diesel trucks have 3.3 gears in them, so did the vehicle I bought that other SD25 turbocharged diesel engine from, I cannot remember the tire size that was on that Volvo Wagon the SD25 was in.