The Cummins test jeep was a late 90s TJ. So it has an AX-15 with a 0.79 over drive and a 1.1 transfer case. It could've had 3.07s or 4.10 gear ratios (depending if it was a 4 or 6 cyl initially).

But otherwise it was stock. Stock tire size and nothing else bolted onto it. Using the R2.8 gear calculator, with 3.07s, at 70 mph, it puts it near the top end on the "efficiency" range of the R2.8. So 3.07s make sense.

Looking at that rig they built, here is what it looks like. Assuming retained pre-TDI LT77 and 1.4 Transfer case:

So yeah, pulling an extra 1100 rpm to do the same as that TJ, and I'm sure its significantly heavier. Bump it up to 85 (pretty much the average speed needed to cross the country on any interstate without getting run over) then you are really pushing that engine at 3700 rpm. Of course it wouldn't be an old rover unless it was screaming along at 3,000 rpm.