I run a 2.47 rear in my 300TD (617) with an A/W intercooler, boost at 16psi, and full load screw turned all the way in. I get about 18 second 0-60... I drive around with it in D most of the time (at around 1800rpm (or lower) on back roads, up around 2700rpm at 75 on the highway, IIRC, its been a while since I have driven my car
), drop it down to S on the steeper hills below 55mph (yielding more than 2400 but less than 3K rpm). I can say for sure I know the engine makes torque well below 2000rpm if you give it fuel, but it won't build max boost until 2400rpm, so I suspect the torque peak is somewhere between 2400 and 3000rpm... Also, I find that if I wind it way up on my 0-60 it doesn't go a whole hell of a lot faster than if I stay under 3100 or so... thats what makes me think peak torque is between 2400 and 3000. Oh, btw, with all the mods I think I am getting BETTER fuel economy... I have to do more testing to make sure.
Another thing: you can make the automatic tranny serve to your benefit with the 2.47 rear if you adjust the shift points via the bowden cable, and the TC acts like a crawl gear!!! I wouldn't even think about putting a 2.47 in back of a manual 4spd unless the gearing was just right. I think I would rather have more like 6 speeds if it was manual.