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Thread: ?: Tire Size Question

  1. #1
    Join Date
    May 2018
    Location
    Phoenix, AZ, USA
    Posts
    5
    My Cars
    2015 BMW 320i

    Question ?: Tire Size Question

    Hello everyone,

    I know very little about cars and am asking for your assistance.

    We are going to be leasing a 2018 BMW 320i and we are trying to decide between 2 cars.

    One of the differences is the size of the wheels and tires.

    Car #1 has tires that are 225/50 R17
    Car #2 has tires that are 255/40 R18

    Would you mind telling me what the difference is?

    From what Ive read it sounds like Car #1 tires are thinner on the pavement, but wider from the wheel to the wheel cut out on the body while Car #2 tires are thicker on the pavement and thinner from the wheel to the wheel cut out. I believe the wheels on Car #2 are 1" larger. Is that correct?

    Also, can you tell me which of the two you would prefer and why?

    Sorry Im not knowledgeable about this and I do appreciate your expertise!

    Oh, also Car #1 has "Park Distance Control". Is that a big deal because Car #2 does not have it.

    best to you,

    Rick

  2. #2
    Join Date
    Aug 2013
    Location
    Seattle, WA
    Posts
    1,433
    My Cars
    1999 BMW M3 Coupe
    Here's what the tire numbers mean:

    First number is the width of the tire tread in mm. So, 255 is 255 mm wide.

    Second number is the aspect ratio representing the ratio of the tire thickness to the tire width. A larger aspect ratio will be a thicker/taller tire, and a lower aspect ratio will be a thinner/shorter tire, though since this is a ratio it changes as the tire width changes. For example, the 255/40 is 90mm thick (225*0.40). The 225/50 would be 112.5mm thick (225*0.50).

    Third number is the wheel diameter the tire is supposed to fit on, in inches.

    In your case, car one has narrower 17 inch wheels and tires, and car two has wider 18 inch wheels and tires. For driving on the street, the only difference will be slightly better gas mileage with the thinner tires. 17 inch tires are also cheaper to buy replacements for than 18 inch. If you want top performance though, the wider 255 tires will have more grip. The 18 inch wheels may also look nicer on the car.
    Last edited by TostitoBandito; 06-26-2018 at 08:06 PM.
    1999 M3/2/5 - Titanium Silver - Track/Weekend Toy


  3. #3
    Join Date
    May 2018
    Location
    Phoenix, AZ, USA
    Posts
    5
    My Cars
    2015 BMW 320i
    Thanks so much TostitoBandito! Super helpful and I appreciate your explaining that to me! Take care and best to you! Rick

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