Tire sizes
 

I want to replace the tires on my 06 3.0, 18" wheels.
Any suggestions as to what size would work?

255/55/18 works for 18" wheels, what's on them now?

Michelin MXV 4 plus, 255/55x18, but can I get a chunkier look?

Sure you can. I am running 275/60 on OEM 18 inch rims with no ill effects, looks good. No rubbing anywhere, doesn't overhang the wheel wells.

My speedometer is slightly over-corrected with these. On that front, I would have been dead on with 275/55 but couldn't find a tire I wanted that size.

Disclaimer: This is NOT what the BMW engineers decided you should have, so I have sinned in doing this. I have chanted ten million times "I did not NEED to do this. No one NEEDS what OEM does not provide." as penance.

JWMich,
Thanks, your X5 is a 4.4 mine is a 3.0, (Sport Package) I am wondering if these are the same wheels?
My concern is mostly quiet, the Michelins on the X5 now are fairly quiet even though they are almost at the wear bars...
The concerns here are not snow conditions...just wet and dry and quiet...(I don't miss the snow and ice at all!!)

honestly, for 'look', all you can do is look around - the Mich 275's might do it for you and we're already used to the speedo thing. I buy and bought Pirelli's for the look and also the long relationship (80's F1) but I am certain the noise would drive you ape-**.
You cain't track 'em so near any name tire that looks right and their owner says isn't noisy will be good for summer/heat.
MOST recommendations will be Michelin and Bridgestone, in my experience. I stopped asking as I don't like their at-limit behavior (with the advent of DSC, not nearly the issue it was, but burned/spun once, twice shy, and I hate 80k mi warranty, plastic-tire sound, much prefer Rubber at .8g, the limit of what these bones can still stand).
Dunno of anyone here that has 'em, but check Dunlop (not certain about noise there and tend toward pricey).
MHO, avoid all-season if you can.
What's on your Porche? You clearly like their take on handling, and they now offer more than one vehicle so is that worth a look..
If I read you correctly, it's time to treat yourself to a pop or two and peruse the 'show us your X5' topic. Near every tire is displayed (for better/worse) there.. then ask the member if they are noisy.