OK, I have a 2011 Xdrive 5.0 Sports Package Adaptive Drive. Came with Style 335 19" x 9" wheels - 255/50/19 tires. I got rid of the run flats immediately and went to Vredestein Wintrac snows. The Vreds were a huge improvement (overall) comparted to the Conti run flats.
fast forward to today
Installed Avante Garde M510 20" x 10" Wheels with Vredestein Ultrac SUV Sessantas.
The wheels weigh 30 lbs each and the BMW Style 335s weigh 36 lbs. The Sessantas are 1.5 (rear) - 2.5 lbs (fronts) lighter than the Wintracs so overall I reduced unsprung weight by 7.5 (rear) - 8.5 lbs (fronts) per wheel.
315/35/20 - rear
275/40/20 front
Driving impressions after taking a 40 minute drive out in some twisty Colorado canyons:
I was surprised. The car immediately felt more grounded -I don't mean cumbersome - I mean more stabile. The reduction in weight was also noticeable - the car felt very quick to respond and jump forward upon acceleration (Another time I reduced an E36 M3s weight by 10 lbs per wheel and that difference was larger) However, the 8 lbs per wheel has given the X (I believe) a slightly more nimble response. Although, I realize this will come at the same time as much wider tires which will have their own characteristics.
The cornering was improved, being extremely flat and solid - more go kart like compared to the 19s. The turn in with the Sessanta summer tires was much quicker as well (to be expected).
Overall the feeling is that the X has become a bit of a sporty brawler, and I like it so far!
Another feeling that makes no sense - the car feels lower to the ground (in a good way). I haven't a very good idea why I would feel this, but it is a distinct impression and if I didn't know better, I would be wanting to conclude that the car was a couple inches shorter. The only thing I can surmise is that the "well grounded-ness given by the wider tires is producing in me the impression that I am driving a car that is lower to the ground.
Seriously, this X with Adaptive Drive is a pretty good cornering machine but the 20s with the sessanta tires has just increased the Xs cornering enjoyment by around 30% to 50% (again, all my subjective perception - nothing measured obviously). But, I am sure the car's real cornering ability has improved as well!
I can't wait to drive her some more tomorrow!
I need to go search some posts her by Ard on getting an alignment (and rhe specs to get dialed in) because I intend to have an alignment done in the am by a reputable shop!
More impressions to come, but I an happy so far and surprised by the increase in handling ability!
