So I'm looking for tires to buy, and I'm fine with paying more money for better tires, and trying not to confuse myself. I know that the aspect ratio is computed with some combination of the width and diameter.

Can I buy a tire just based off the diameter?

  • 1
    did you try google? "tire calculator" produces literally thousands of hits. Not to mention every major retailer has such explanation on their web sites.
    – agentp
    May 16, 2017 at 19:32

1 Answer 1


As an example, my tire size is 235/45/18. This means the tires are 235mm wide, have a 105.75mm total side wall height (235mm x 45%), and they fit over an 18" Ø wheel.

You need all these numbers, because a 305mm tire width would not fit my wheels, nor would a 195mm tire width. The second number (45 in my example) significantly influences ride comfort and tire noise, and can potentially cause your tires to rub on the fender if spec'd incorrectly.

I would recommend sticking with whatever the manufacturer recommends - you can find this on a sticker on the driver's door, in the owners manual, or on any number of websites. If it's a relatively modern car, there should be plenty of options from reputable manufacturers (ie Bridgestone, Toyo, Pirelli, Falken, Michelen, Goodyear, Yokohama).

  • thanks alot, this is exactly what i was looking for.....buying things off the internet can be pretty complicated if you care about what your buying....I was thinking that the last number was tire diameter but it's actually wheel diameter May 16, 2017 at 18:21
  • Hmmm, for me, a wheel is the entire thing, rim plus tire, and the last number is the diameter of the rim...
    – sweber
    May 17, 2017 at 4:46
  • @sweber check the categories on these sites discounttire.com, tirerack.com, lesschwab.com May 17, 2017 at 16:16
  • I see. Does this mean that the term 'wheel' is used a bit sloppy in this context? Because the last number definitely is the diameter of the rim, not the outer diameter of the tire.
    – sweber
    May 17, 2017 at 17:58
  • @sweber rim is slang for wheel. Can you point out where the sloppy use of the word wheel is? I don't see it. May 17, 2017 at 18:32

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