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Tyre size help

Discussion in 'General 4x4 Discussion' started by mcflurry, Nov 23, 2018.

  1. mcflurry

    mcflurry New Member

    Hello all,

    I'm looking at purchasing a set of BFG KM3's for the GU and having a difficult time with tyre sizing.
    The two I'm looking at are the 285/70R17 LT and the 35X12 5R17 LT.
    I understand the former size, being 285 width, 70 ratio etc, but what the hell is 35X12 and why is it not expressed just like all the other tyres?

  2. Aza013

    Aza013 Well-Known Member

  3. silkwood

    silkwood Active Member

    Your 285's are what is generally called "33's" (33" diameter- actually around 32.7"). The 35x12.5's are as the label states- 35" diameter (12.5 width) to suit 8.5" - 11" width rims.

    Why? 'cause Merica...

  4. abw

    abw Well-Known Member

    285/70R17 is a 33" tyre (285mm wide) and the 35X12 5R17 is 35" (12.5" wide).
  5. Colly18

    Colly18 Active Member

    Hi Brad,
    I assume that apart from gaining some extra height/ground clearance you are well aware of the negatives that go with increasing tyre size? Many (myself included) opt for a modest tyre size increase (say 1-2 inches diameter that will give you an extra 1/2 to 1 inch ground clearance) and even that will probably come with a vehicle acceleration and fuel economy penalty. Jump from a standard 31 inch to a 35 inch and you will probably be cursing about a lack of acceleration and poor fuel economy (unless you are going down the track of diff gearing changes to compensate?). The 285/70R17 as an increase from a standard 275/65R17 seems like an acceptable way to go IMO, remembering your speedo will be out 5%.
    Last edited: Nov 23, 2018
  6. dno67

    dno67 Well-Known Member

    The 285s should have a better load rating,
    something else worth considering.
  7. mcflurry

    mcflurry New Member

    Thanks for all the help gents, much appreciated. I hadn't even considered load rating @dno67 that's well worth considering. Also hadn't given enough thought to acceleration @Colly18 so I appreciate that. I don't want to mess around with diff gearing as my budget is starting to stretch thin after all the recent mods.
  8. Triton14

    Triton14 Active Member

    Also affects your braking as well!

    On the speedometer, it actually would bring you close to your actual speed, most speedometers are set 5% under the speed limit but its an interesting subject on how speedo's are calibrated between brands & how accurate are they??

    I went up from 70's to 75's & gave me some extra clearance which I did enjoy & apart from the fact they were frigin awesome offroad :D
    I just make sure I am aware of the impact it has on the vehicle on the tar!
    Last edited: Nov 27, 2018
  9. Rhett HS

    Rhett HS Active Member

    It should read "35x12.5R17".
    35" overall tyre diameter.
    12.5" section width.
    17" rim.

    In 'metric' that equates to 315/70R17.

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