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Thread: Tire Sizes

  1. #1
    bill57 is offline Cadillac Owners Fanatic
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    Tire Sizes

    Is there any downside if I use a 205/70-15 tire in lieu of the specified 205/75-15 size?


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    Submariner409's Avatar
    Submariner409 is online now If it won't run, chrome it
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    Re: Tire Sizes

    15" ??? What year car is this ??? use the "User CP", up in the blue bar ^^^ to put your car, year, mileage in your login header.

    Go to www.tirerack.com and look up the specs on a good all-season tire for your car's original tire size - on the driver's door sticker ? Then look up the specs for the new tire. If the outside diameter of the two tires is within 0.3 inch or so there will be minimal effect on speedometer or handling performance.

  4. #3
    bill57 is offline Cadillac Owners Fanatic
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    Re: Tire Sizes

    According to the specs. from tirerac.com, the 70 series is 0.8 inches larger than the 75 series. So, I guess my speedometer will show the speed a bit faster than true speed. Any other downside?

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    johnny kannapo's Avatar
    johnny kannapo is offline Cadillac Owners Connoisseur
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    Re: Tire Sizes

    The 205 is still in the ball park for diameter. 70 verses 75 aspect ratio (width) not a huge difference there either but if you are buying new get the proper size for the rim width. The side wall will then be correct and the bead won't be pinched in by too small a rim width assuring expected tire performance.

  6. #5
    Submariner409's Avatar
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    Re: Tire Sizes

    If you go to the 70 and it is 0.8" larger (outside diameter) then it will roll farther in one revolution than the 75, so the speedometer would then read slower than actual car speed.

    Like johnny k posted, keep an eye on proper rim width.

  7. #6
    bill57 is offline Cadillac Owners Fanatic
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    Re: Tire Sizes

    Thanks for the information. I'll probably stick with the correct size.

  8. #7
    sls2004 is offline Cadillac Owners Member
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    Re: Tire Sizes

    Quote Originally Posted by bill57 View Post
    Is there any downside if I use a 205/70-15 tire in lieu of the specified 205/75-15 size?
    The 70 or 75 in the tire size stands for the approximate percentage of sidewall versus the tread width. A 205/75-15 has a sidewall height of 205x.75=153.75. A 205/70-15 has a sidewall height of 205x.70=143.5. When you go down in sidewall heigth you have to go wider in tread width to keep the same approximate tire heigth. If you go to a 70 series tire you can go up to a 215/70-15, 215x.70=150.5. This tire will be slightly smaller in diameter but the extra 10mm in tread width should not be a problem.


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