I purchased a 03 MCS with 10K on it....mainly for daily driving. I love the car but not too sure about the size of the wheels. I like the 17" or 18" look better. Can I slap on 17" and 18" wheels and not have to do anything else to my suspension or cambers and would I have issues with mushrooming? What is mushrooming exactly anways?

Thanks
Noob

 

Assuming you have stock suspension, You can use 17" or 18" wheels provided-
You use wheels that are 4x100 bolt pattern and up to about 8" wide
Wheel offsets about +38mm to +48mm (+42 to +45mm is safest)

Lowered suspension leaves less tire clearance, keep that in mind.
Stock tire diameter is about 24.4" tall so tires of that size or slightly smaller are fine.

For 17" rims for no rubbing issues
205/45-17
215/45-17

For 18" rims
215/35-18

Mushrooming is deformation upward of the front strut towers seen from the engine bay due to hitting road hazards or potholes and transmitting the shock/energy up to the top of the towers causing damage.

You can avoid mushrooming by driving carefully and not hitting any potholes especially at speed. Using larger rims usually means lower profile sidewall tires which are stiffer and transmit damage more easily when hit both to the rims and to the suspension system.

You can add M7 Strut tower plates to help to reinforce the strut towers and some owners add strut tower bars to perform a similar function.

Be aware that 17" and 18" wheels are often heavier than 16" rims of equal design. 45 series tires are not too bad for street use but 35 series tires aren't as forgiving. They tend to be good for handling but harsher riding and sometimes noisy. Pick your tires carefully. You can consult with Alex@tirerack.com for more info.

 

minihune nailed the answer.

One addition to his post would be the inclusion of the 215/40-17 tire size. Not as forgiving as a 45 profile tire (still mich better than the 35 profile), but the size is more acceleration friendly than the 215/45-17.

 

Yes, 215/40-17 is a no rubbing option but runs a little less than stock tire diameter so speedo and odometer read a bit higher than actual speed or mileage. Not that big a deal.

40 series sidewalls are stiffer than 45 series so don't use them on rough pothole ridden roads. If your roads are very smooth then it's fine. If you tend to drive corners fast, don't mind a slightly harsher ride, like a more aggressive look with 17" rims, and want to accelerate (due to lowered gearing of the smaller tire diameter) quicker then this is a viable tire size. It's not well suited for most owners looking for daily street driving.

215/40-17 tend to weigh a little less but cost alot more for a given tire compared to other sizes in 17".

225/45-17 is also possible but it's a little heavier and a little taller than stock at 25" so rubbing is more of an issue for some. Using stock suspension it's close, just don't overload your MINI or carry four people all the time. A little rubbing is possible but not that bad. You can let it rub and the inner wheel arch plastic will wear itself down along with a little of the outside edges of your rear tires. Tire selection is very good in 225/45-17.

 

not to forget 225/40/17's (nitto's)... to stir it up a little more.