Diff swap time! Ratio questions.

My 535/5 has an open 3.46 diff. Especially now with the Mark D chip installed, this car needs a LSD.

I am torn on which ratio to persue though. The 3.15 or 3.23 promise to drop highway rpm, the 3.73 or 3.91 promise to bring back some oomph lost to altitude, and probably age. Most of the time I can cruise around 65-70, limit is 60 for most of my highway drives. I do have a long road trip coming up, and cruise will likely be more like 80-85. I know the engine can easily handle the sustained rpm but what happens to economy at a high rpm cruise?

Anybody with a 3.91 with experience to share? I think 3.73 would be ideal but those seem to be a little harder to find.

Mine has an 25% LSD in it. Has plenty of pep. I'm not sure which one it is, but I can try to look for ya as I'm changing the oil a bit later today.

If I had to choose a diff myself I would throw in the same one that's in there right now. In overdrive (4th) at 120km/h the engine sits at about 3k rpm (I think, don't quote me on that). If it was any higher then this I think cruising down the highway would be a bit uncomfortable.

Haven't had time to look at it yet but my car does 120km/hr at pretty much exactly 3k rpm (in top gear with stock tires/rims). You can probably figure out the ratio from that...