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  1. #1

    Default Tire Sizes the same?

    Right now I have P235/75/R15 tires on my truck now, my friend has some tires that are 30x9.50r15. How close are they in size? Will it throw off my spedo alot? Please dont laugh cause I dont know anything about tire sizes.
    Matt
    1987 GMC Sierra Classic 5.7TBI


  2. #2
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    The 30x9.5x15 are about 1.12"'s bigger than your current tires resulting in about a 2.5mph diffrence in your speedo. So if your doing 60 your speedo is only reading 57.5. Hope this helps

  3. #3

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    Quote Originally Posted by finalday7 View Post
    The 30x9.5x15 are about 1.12"'s bigger than your current tires resulting in about a 2.5mph diffrence in your speedo. So if your doing 60 your speedo is only reading 57.5. Hope this helps
    that answer was PERFECT, the rim width is the same right?
    Matt
    1987 GMC Sierra Classic 5.7TBI


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    Are you getting new rims with the tires or are you just using the same? Either way they will fit.

  5. #5

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    using the same rims
    Matt
    1987 GMC Sierra Classic 5.7TBI


  6. #6
    Master Mechanic TahoeFever's Avatar
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    Quote Originally Posted by finalday7 View Post
    The 30x9.5x15 are about 1.12"'s bigger than your current tires resulting in about a 2.5mph diffrence in your speedo. So if your doing 60 your speedo is only reading 57.5. Hope this helps
    Officer I was only doing 57.5 in a 55 zone!
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  7. #7
    Legend unplugged's Avatar
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    Quote Originally Posted by MD4x4 Fireman View Post
    Right now I have P235/75/R15 tires on my truck now, my friend has some tires that are 30x9.50r15. How close are they in size? Will it throw off my spedo alot? Please dont laugh cause I dont know anything about tire sizes.
    If you use the tire size calculator at redrock4x4 you get this:
    Result:
    235/75/15 = 28.9x9.3/15


    If you use the online calculator at onlineconversion.com
    You will see that if you speedo shows 60mph you will actually be traveling at 62.3mph
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  8. #8

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    One thing most do not consider is that bigger tires require more power at all speeds, present a larger profile to the wind at speed. They cost you in lost fuel mileage and more wear and tear on suspension parts but yes they do look cool!!
    Happy 2 Wheeler!

  9. #9

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    im just adding them so i have traction, i plan on buying a cheap set of rims from the junk yard to use for them so if i wanna go offroading or when it snows.
    Matt
    1987 GMC Sierra Classic 5.7TBI


  10. #10
    Legend unplugged's Avatar
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    Quote Originally Posted by FireRed View Post
    One thing most do not consider is that bigger tires require more power at all speeds, present a larger profile to the wind at speed. They cost you in lost fuel mileage and more wear and tear on suspension parts but yes they do look cool!!
    When I dropped from 305's to 265's I noticed much better pulling power and it is much easier to turn at low speeds. Of course, you can change the gearing to improve pulling, but that is expensive with 4x4 since both diffs have to be the same ratio. When increasing tire sizes more than 3% the increased loading on the brakes is dangerous.
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