I'm looking to put new tires on my xiT and am considering going with a wider profile such as 255/45-17. I have a set of 8X17 wheels off of a 530 sport that fit OK with the stock 225/50 tires and even thought the offset is 20mm vs. the stock wheel @ 43mm they fit with plenty of clearance. The diameter is +.2" so speedometer error is negligible, and the cross section goes from 8.9" to 10". Has anyone done this and what should I be looking for in terms of any potential problems?
Your owner's manual should list recommended tire sizes Generally speaking, you should be able to go with a tire 10mm wider than your existing tire. You can do a little more by going with spacers, which will push the wheels out further to make room for a wider tire by the same amount as the thickness of the spacers. I don't recommend getting TOO ambitious with treadwidth, as you may risk hitting the top of your wheelwell or worse, some very expensive suspension hardware. Don't fall into the misleading trap of thinking that a wider tire necessarily means more traction. Traction is a function of your car being able to put its power to the road, and there are a lot of other factors that go into this, such as the rubber compound used, the tire's temperature range, as well as what type of tread it has. Also, by putting on a wider tire, you're increasing the weight of the tire and thus, the overall weight of the wheel and tire together. This creates a greater workload for your rear suspension, brakes, front and rear control arms, as well as your axle shafts on your carrier. BMW had good reason to not simply stick the widest set of wheels and tires under the car and calling it a day. It's called balance and handling, and this isn't always accomplished with the widest set of tires available. Depending on what you're trying to accomplish, good luck with what you are setting out to do.
