tyre sizes

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Sweetlips
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tyre sizes

Post by Sweetlips » 13 May 2019 15:14

So i've been shopping around for a set of 33's for my 17" rim and it came to my attention various people has varying interpretations.

Maybe someone here can assist me. I was always under the impression that a 285 is the equivalent to a 33" tyre, well that isn't correct.

Some refer to 33/12.5/R17, others to 285/70/R17 and others 305/65/R17.

Obviously the different sizes has different prices, what i found so far is that the 33/12.5/R17 is available in BFG, Cooper or MT and obviously vrek expensive.

Any advice/guidance gents.
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Re: tyre sizes

Post by iandvl » 13 May 2019 15:39

Lawrence,

These all differ.

33/12.5/R17
17" rim.
12.5" rim width (ie: 12.5J).
33" diameter of tyre.

285/70/R17
17" rim.
285mm tread width.
profile is 70% of tread width ie: 70% of 285 = 199.5mm

So, 199.5mm * 2 + 431.8 = 32.7" (close to 33").

305/65/R17
17" rim.
305mm tread width.
Profile is 65% of read width. ie: 65% of 305 = 213.5mm

So, 213.5mm * 2 + 431.8 = 33.8 " (slightly larger than previous option).

Of the three, I'd recommend one of the latter two tyre sizes - 285/70/R17 or 305/65/R17.

Reason being is that Patrol stock rims are 8JJ, if I recall correctly. In short, I presume the 33/12.5/R17 tyres will be a little large for the stock rims.

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Re: tyre sizes

Post by Sweetlips » 13 May 2019 16:51

Thanks Ian

You would expect some of the tyre dealers to know this and advise accordingly.
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Re: tyre sizes

Post by Tinus lotz » 13 May 2019 18:11

Jip like Ian said .....size you want for your car is 285 70 17
285mm is how wide it is 70 percentage of how wide it is for the sidewall ..... the 33 is the size in inches and 12.5 the widest part

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Re: tyre sizes

Post by Peter Connan » 14 May 2019 07:01

One small correction: the width of a tire (both metric and imperial) is measured at the widest point (ie across the sidewalls) not at the tread. Tread width is typically 20-30mm less. And of course this changes with rim width, so it is basically a theoretical value.

Actually all these values are nominal or theoretical, not actual installed.

I would say a 33 x 12.5 would fit well on an 8" rim. I have hed them on 6.5" rims, but that was 15" not 17", so a lot more side-wall height.

Personally I prefer to fit tires on a rim that is a bit narrower than the theoretically ideal as that protects the side-walls better. Theoretically there is a resulting loss in stability at higher speeds, but I have never been able to tell the difference.
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Re: tyre sizes

Post by davidvdm » 14 May 2019 09:05

Peter, thanks for that info. I have been told on the "other" forum, that my 33x12.5x15 is too wide for my 8J rims. Wanting to make it worse with the 35x12.5x15's at some stage. I must say, I can feel the difference in handling (sidewall flex) between the old 29" and 33" rubber. And that is after I went from 7J to 8J rims.

Someone did also mention that a narrower rim would aid in preventing de-beading ay lower pressures, which is a bonus if you can't afford beadlock rims.
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Re: tyre sizes

Post by iandvl » 14 May 2019 09:13

Peter Connan wrote:
14 May 2019 07:01
...
One small correction: the width of a tire (both metric and imperial) is measured at the widest point (ie across the sidewalls) not at the tread. Tread width is typically 20-30mm less. And of course this changes with rim width, so it is basically a theoretical value.
...
I always assumed it was the width of the tread... I see now I was entirely incorrect. Width of widest part of the tyre. Including any nubblies / manufacturer lettering and similar on the outside...

Thanks - did not know this. :salute:

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Re: tyre sizes

Post by Peter Connan » 14 May 2019 10:01

davidvdm wrote:
14 May 2019 09:05
Peter, thanks for that info. I have been told on the "other" forum, that my 33x12.5x15 is too wide for my 8J rims. Wanting to make it worse with the 35x12.5x15's at some stage. I must say, I can feel the difference in handling (sidewall flex) between the old 29" and 33" rubber. And that is after I went from 7J to 8J rims.

Someone did also mention that a narrower rim would aid in preventing de-beading ay lower pressures, which is a bonus if you can't afford beadlock rims.
Yea but hang on, you are now comparing a 9" high side-wall with a 7" high side-wall. That's going to have a significant difference in stability.

Standard rim width for 33 x 12.5 x 15 is 8.5 - 11", so you are pretty close. I am sure they will be fine.

Just for reference, I built a bush-only game-viewer type vehicle using mostly Beetle running gear many years ago. A friend with a Sani gave me some cast-off tires for it. If I remember correctly, they were 265/75 R15s. I mounted those straight on the standard Beetle rims, which were about 4" wide. It looked really weird, but it worked pretty well (although they were a bastard to get onto those rims). And I never had a sidewall puncture, or a de-bead. But the whole car only weighed about 750kg. I did once run the pressures so low that a tire rotated on the rim under braking and tore the valve stem off the tube...

At somewhere around 80km/h, that did give a pretty unsavory death wobble, but no stability issues.
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Re: tyre sizes

Post by SJC » 18 May 2019 06:52

Sweetlips wrote:
13 May 2019 15:14
I was always under the impression that a 285 is the equivalent to a 33" tyre, well that isn't correct.
:think: 285/70r17 is in theory 32.8” but then there is also slight differences in size between the differrent brands:
viewtopic.php?f=49&t=6211
https://tiresize.com/calculator/

285/70r17
Firestone Destination M/T 32.8x11.5(292)
Maxxis Bighorn 33x11.7(297)
Hankook Dynapro M/T 32.71x11.22(285)
Kumho Road Venture MT KL71 32.7x11.2(284)
Cooper Discoverer STT PRO 33.03x11.5(292)
Cooper Discoverer ST MAXX 32.99x10.7(272)
BFGoodrich Mud-Terrain T/A KM2 33x10.7(272)
BFGoodrich All-Terrain T/A KO2 32.7x11.5(292)

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Re: tyre sizes

Post by martyn » 18 May 2019 13:24

Just to make a correction on a previous comment. 33*12.5*17. 33 refers outer diameter of the tyre in inches
12.5 is the width of the tyre in inches (not rim width)
17 is size of rim diameter in inches
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