[GTBrandon]

I can't stand these Continental Run-Flats that came with the car, it's time for an upgrade. I plan on keeping these tires since they still have tread and using them as winter tires while getting myself a new pair of summer tires. I do not plan on going run-flat as they seem to add noise and decrease ride comfort, and therefore have chosen Michelin Pilot Super Sports, but here is my dilemma. I find it odd that this car has stock 225/50R18's, as this seems like a pathetically small contact patch for a sport-oriented car. I had bigger stock tires on my Acura TL honestly. Anyways, I was thinking of slightly plus-sizing the contact patch in order to gain a little more traction and possibly decrease the side walls. I was wondering if anyone had experience with this and could offer any advice. It seems that my options are as follows:

235/50R18
235/45R18
245/45R18


I need to check to make sure all of these would sit properly on the rim, but I was hoping someone could shed some knowledge on this subject.

And before you say just get new rims, I wish I could upgrade to those beautiful 19inches but I don't want to spend the money right now so it is out of the picture.

Thanks!

[Billfitz]

You need to check the outside tire diameter. Typically if you go one size wider you go one aspect ratio smaller to maintain the diameter. Going from a 225-50 probably means going 235-45, 245-40.
The RFT don't bother me, they're no worse than the Michelin Primacy I had on my Altima. However, I have 225-55/17. RFT tends to be more of a problem with lower aspect tires, as there's less sidewall.

[GTBrandon]

I'm just wondering if anyone else has done plus sizing on this car and what they experienced afterward. I do believe it would be better to have a wider contact patch and therefore more grip, but do not want to affect ride quality negatively. Also, I want to make sure that changing the tire size would not disrupt any of the electrical systems that are connected to the tires ie: adaptive suspension, steering, abs, etc.

[Billfitz]

Originally Posted by GTBrandon

but do not want to affect ride quality negatively.

The less sidewall the stiffer the ride. One of the reasons why RFT get a bum rap is the large wheel/low aspect ratio rage, so you the double whammy of extra sidewall stiffness from the RFT design plus less shock absorption from the shorter sidewall. I would think that if you went one aspect ratio lower while going to a non-RFT that it would balance out. Going two aspect ratios lower might not. Also, wider tires have higher rolling resistance, which reduces fuel mileage.
As for the rest, look at what's available as far as wheel/tire options for the car with the suspension you have. Anything you can order from the factory would have to be OK.

[lethbridge]

Suggest you use a tool like this one (if you haven't already) to work out the impacts of any change:
http://www.rimsntires.com/specspro.jsp

I can't comment on specifics as I have 19" wheels. But I can confirm there is no issue with comfort with runflats on 19" wheels compared to earlier 3 series etc as they increased the overall wheel diameter (my rears are 255/40/19)... Whatever you do you will not be wanting to change the overall wheel diameter too much (or you will get speedo inaccuracy) so the sidewall height should remain about the same - if you don't know 225 is the width in mm and 50 = the sidewall is 50% of the width = 112.5mm. So a 245/45 = 110.25mm (noting the difference in overall diameter will be x2 - i.e. 4.5mm smaller)...

Not sure it is really worth the fuss, cost and impact on mpg & insurance (its a mod - especially ditching runflats & in theory you should notify them). Keep in mind that they keep fronts at 225 even on 19" so you need to be sure on clearances and any xdrive impacts if you do change the fronts... Given the xDrive I'd be surprised if you could notice any difference in grip unless you are doing timed laps on a track... The monumental torque on my 335d won't unsettle 225s on the front 255s on the rear unless severely provoked and in sport +... Having said all this everyone seems to rave about the MPSSs...

[Richbot]

I considered doing a 245/45-18 but didn't because there was a great deal on OEM pirellis.

I would not worry about using any of the sizes you mentioned. Plenty of room and as long as they are all the same the car won't care.

[GTBrandon]

Originally Posted by lethbridge

Suggest you use a tool like this one (if you haven't already) to work out the impacts of any change:
http://www.rimsntires.com/specspro.jsp

I can't comment on specifics as I have 19" wheels. But I can confirm there is no issue with comfort with runflats on 19" wheels compared to earlier 3 series etc as they increased the overall wheel diameter (my rears are 255/40/19)... Whatever you do you will not be wanting to change the overall wheel diameter too much (or you will get speedo inaccuracy) so the sidewall height should remain about the same - if you don't know 225 is the width in mm and 50 = the sidewall is 50% of the width = 112.5mm. So a 245/45 = 110.25mm (noting the difference in overall diameter will be x2 - i.e. 4.5mm smaller)...

Not sure it is really worth the fuss, cost and impact on mpg & insurance (its a mod - especially ditching runflats & in theory you should notify them). Keep in mind that they keep fronts at 225 even on 19" so you need to be sure on clearances and any xdrive impacts if you do change the fronts... Given the xDrive I'd be surprised if you could notice any difference in grip unless you are doing timed laps on a track... The monumental torque on my 335d won't unsettle 225s on the front 255s on the rear unless severely provoked and in sport +... Having said all this everyone seems to rave about the MPSSs...

Thank you for that website, great tool for speccing out different sizes! The thing is that you have wider tires on the back and therefore have fewer issues with fishtailing out, especially considering the rear-bias xDrive system. My car has the same 225/50R18's all around and is why I was considering plus-sizing. Also unfortunately they do not make SS's in 225's, only 235/50R18's. I'll have to do some more research on fitting everything in without going away from stock suspension and body panneling. But will contact BMW and update with what they say in regards to fitting as well as possible voiding of warranty.

[liveevil]

My staggered GT came stock 225/50/18 front and 255/45/18 rear, and I fitted 245/45/18 on the front with no issues.