Wheel spacers needed?
 

...when switching from the standard 19" OEM alloys on my 3.0d Sport to OEM 17" alloys (Style 57)? I have to spend 10 days driving about in Scotland from next week so I'm finally going to have to stick my winter wheels on! (Vred Wintrac Ext 4's). Also, there's a massive difference in the two wheel sets so is it necessary to muck about with the geometry at all?

Btw, in the recent few days worth of icy/snowy weather in Surrey my rear drive 911 on winter tyres was a LOT more effective than the E53 on (admittedly quite worn) Michelin Diamaris..! Goes to show how good the winter rubber is.

You don't need spacers... More of an aesthetic thing...

No need to "muck about" with the alignment either. :)

Great. So life is simple for once!

You didn't say what size tires you have mounted to either set...but if you're running the BMW recommended tire size...then regardless if you are running 17"/18"/19"20" wheels...the overall diameter/circumference of the wheel/tire is basically the same for whatever sized wheels you have.

The whole process of upsizing/downsizing wheels is that you maintain the overall diameter/circumference (within 3% +/-) to maintain ABS/DSC/speedometer/odometer:

I drove a 2009 Porsche for about 2hrs in a snow/ice storm with summer tires, and it was the most terrifying drive I've ever had. Rear engined Porsches can be awesome in snow, but the wrong tires will kill any car's chances in the snow. Snow driving in a 911 reminds me of a certain 911 Carrera with "Let's Boogie" replacing the "Carrera" script on the sides, (anyone remember John Buffum?? )

Summer tires and snow - I mean snow like complete snow covered roads - will never work. Not in a Porsche and not in an X5. You will not be able to drive out of your parking lot ... also not on a flat road.