PSS 235/40 vs. 45
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    1. #1
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      Greater Boston
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      PSS 235/40 vs. 45

      Not very knowledgeable about tire sizes. My MY16 S60 with sport chassis came with wheels/tires in 235/40/19. Swapping for 18" Titania's with PSS. Local tire shop says my new 18" tires need to be 235/45. What is the difference? Will one handle better? I want a good grip.

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    3. #2
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      Quote Originally Posted by mbd23 View Post
      Not very knowledgeable about tire sizes. My MY16 S60 with sport chassis came with wheels/tires in 235/40/19. Swapping for 18" Titania's with PSS. Local tire shop says my new 18" tires need to be 235/45. What is the difference? Will one handle better? I want a good grip.
      A brief lesson in tire size: for a 235/40-19 tire, the 235 is the width of the tire in millimeters. The 40 is called the aspect ratio and is the distance from the outside of the tire to the inside edge where it mounts on the wheel rim, but it is not expressed in distance -- instead that 40 means the height of the tire sidewall is 40 percent of the tire's width (235mm). The 19 is the diameter of the rim.

      If you are changing the size of your rims, like you are doing going from 19" to 18" rims, you will need to increase the sidewall height by a total of an inch if you are going to maintain the same tire circumferance. This is important, because your speedometer is calibrated to the size of the tires that came standard on your car. When you start changing tire sizes, you can change the circumference of your tires, which can result in speedometer error depending on how big the change is. Here is a useful calculator: http://tire-size-conversion.com/tire-size-calculator/

      The tire people are recommending you keep the same tire width (235mm) but increase the aspect ratio (the sidewall will be 45 percent of the width of the tire on the 18" rims instead of 40 percent of the width of the tire on your 19" rims). The increased sidewall height would compensate for the inch of wheel diameter you lose going from 19" wheels to 18" wheels.

      What you will get is a slightly better ride and better rim protection on your 18" wheels, which considering the beating Boston roads take each winter, is a good thing (people often complain of damaged rims on bad roads with 40 aspect tires). It will still have plenty good handling -- 235/445-18 is still considered a low-aspect tire (the advantage of low-aspect tires is less tire deformation as you go around a corner quickly, but (as mentioned above) it comes at the cost of ride and rim protection).

      HTH!

    4. #3
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      Wow. That was so perfectly explained. Thank you so much. You should make educational videos.

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    6. #4
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      Quote Originally Posted by mbd23 View Post
      Wow. That was so perfectly explained. Thank you so much. You should make educational videos.
      Thank you. Helps that I write for a living...

    7. #5
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      Sure does.

    8. #6
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      Norwegian Blue,

      OhKayyy... Wow, that explanation is concise and clear! Thanks!

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