I don't think you ever go into 6th gear - you do it in 5th, and 6th is not a serious gas mileage gear it's like a 300 RPM difference from 5th to 6

That is basically the point I was trying to make----top speed is going to happen in 5th just as likely as it is in 6th in a 4.8is, so the addition of 6th gear is not going to be some massive advantage in a 4.8 vs a 4.6. 5th gear ratios and final drive ratios are identical in 4.6 vs 4.8. Again--the point is not pegging 4.6 vs 4.8---just using it as a frame of reference.

but the gearing is different, 5th is longer in the 4.6 than in the 4.8 b/c you don't have a 6th gear, which means you are at a RPM that makes more power in 5th at those speeds in the 4.8 versus the 4.6

No, in both a 4.6 and a 4.8, the 5th gear ratio is .08:1, and the final drive is 3.91:1.....absolutely identical gearing when in 5th.

In the 4.8, 6th is 0.69:1, but as you and I both mentioned, that much reduction is going to be a liability rather than an asset in trying to reach top speed with 350 hp.

Bottom line is, both trucks are going to make top speed in 5th, and 5th is identical---is 10 hp going to be good for another 10 mph (to get to the speeds the OP is talking about doing)? I say no, but I've certainly been wrong before.

interesting, I thought the final drives were different, but obviously not...anyways, I know that at an indicated 160 it still felt like there was room to go

I understand----it's not a big deal at all to me either way....just enjoying the discussion. Keep in mind that most BMW's seem to show 5-10 mph of generous error at high speeds when compared to GPS/radar measured speeds, so that could explain what you felt.

also, where'd you hit an indicated 160---somewhere between N. FL and DC? Sounds like a road I need to visit! :D

Scan from my old window sticker. Hope this wasn't too large for this venue, tell me if it is. Look in bottom left corner.

http://i43.tinypic.com/2rpee5j.png

Now thats what I'm talking about! LOL. Unlimited TOP SPEED!

I did that on I-75 northbound heading back from Tampa...3 or 4 lanes perfectly flat and straight and minimal traffic, it was just begging to be opened up :)