"Roadworthy" tyre sizes for NSW

Submitted: Wednesday, Sep 24, 2003 at 16:23
ThreadID: 7404 Views:6009 Replies:3 FollowUps:3
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I have, for some time now, been on a quest to make sense of the ADR and VSI regulations regarding allowable sizes for replacement tyres. There have been previous threads here on the subject. In particular, I wanted to resolve some of the ambiguities in the regulations that result from the use of generic tyre size designations such as 245/70/16, rather than actual measurements in millimetres.

To retiterate, the regulation allows you to fit a wheel/tyre combination that is no more (and no less) than 15 mm larger (or smaller) in overall diameter than the largest wheel/tyre combo indicated on the vehicle's tyre placard. Adhering to this limit means your vehicle will remain roadworthy (vital for continued insurance cover) without the need to obtain a prohibitively expensive engineer's certificate. The problem is that tyre placards refer only to the generic size - 245/70/16 so how do you know what is 15 mm larger or smaller, and larger or smaller than what?

My investigations with tyre manufacturers revealed a significant variation in actual overal diameter despite all being labelled the same size. There are a number of reasons for this that can be important for your choice of tyre (tread depth being one), but not for the theory of "how big can I go?". These differences are allowable under the ADRs which allows for a +/- 3.0% manufacturing variation in the standard (or theoretical) tyre size.

The theoretical size is calculated by [(width x profile% x 2) + rim size in mm]. For example, a 245/70/16 is [(245mm x 0.70 x 2) + 407] = 750 mm. Note, the profile is the height of the tyre, rim to tread, expressed as a % of the trye width. A 16" rim is 407 mm and a 15" rim is 381 mm in diameter. The ADR manufacturing variance means a 245/70/16 tyre may vary in actual overall diameter between 727.5 mm and 772.5 mm - a 45 mm difference!

To cut to the chase, I have been in contact (several times) with the NSW RTA and have written confirmation from them that to calculate the maximum allowable overall diameter for a replacement tyre under VSI 9, you can add the 15 mm to the maximum tyre size allowed under the ADR manufacturing variance. So, for example the 245/70/16 tyre has a max ADR allowable size of 772.5 mm (as shown above), to which I can add 15 mm under VSI 9 and get a maximum allowable overall diameter of 787.5 mm. I can then find and fit any tyre (remembering there are also width increase limits of 26 mm) that is less than or equal to 787.5 mm and still remain "roadworthy" without an engineer's certificate.

Hopefully this may assist some of you when next deciding on what tyres to purchase.

BTW, it has been suggested that one could extend this argument to mean that, because of the above range of actual diameters, and the resultant overlap in diameters between tyre sizes, if I could find say a 265/75/16 that was equal to or less than the maximum calculated diameter, I should be able to fit ANY 265/75/16 tyre no matter what it's actual overall diameter. I did not persue this argument with the RTA (and think it is tenuous, at best) but ....

Smile, you're on ExplorOz
Rohan (Sydney - on the QLD side of the Harbour Bridge)
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Reply By: Member -BJ (Sydney) - Wednesday, Sep 24, 2003 at 16:32

Wednesday, Sep 24, 2003 at 16:32
Geez Rohan, you wanna run that by me again :.)Regards Bob
Where to next
AnswerID: 31895

Follow Up By: Member - Rohan K - Thursday, Sep 25, 2003 at 10:51

Thursday, Sep 25, 2003 at 10:51
Which bit? Smile, you're on ExplorOz
Rohan (Sydney - on the QLD side of the Harbour Bridge)
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FollowupID: 22795

Reply By: Phil G - Wednesday, Sep 24, 2003 at 16:39

Wednesday, Sep 24, 2003 at 16:39
Rohan,

Interesting argument, and I like your conclusion. Do you have a reference for the ADR variance (on the web, preferably).

BTW, I think the 26mm refers to "track" and not tyre width, so rim offset is taken into account.

Cheers
Phil
AnswerID: 31896

Follow Up By: Member - Rohan K - Thursday, Sep 25, 2003 at 17:33

Thursday, Sep 25, 2003 at 17:33
Phil, the ADR is ADR#23. I don't have a web reference but was told about the +/- 3.0% by 2 manufacturers.

The VSI9 can be found a href="http://www.rta.nsw.gov.au/registration/downloads/vsi09.pdf">here

The 26 mm (actually 25 mm) refers to track and offset but you can also fit wheels that are up to 26 mm wider than the largest specified.Smile, you're on ExplorOz
Rohan (Sydney - on the QLD side of the Harbour Bridge)
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Reply By: Graham & Ann - Thursday, Sep 25, 2003 at 17:19

Thursday, Sep 25, 2003 at 17:19
So what tyres did you fit to the Pathy, Rohan?1999 Pathfinder
17ft Evernew 'All Roads' van
AnswerID: 32002

Follow Up By: Member - Rohan K - Thursday, Sep 25, 2003 at 17:37

Thursday, Sep 25, 2003 at 17:37
Graham & Ann, I haven't yet, but are leaning very strongly toward the Cooper ST 245/75/16. Hopefully by the time I take the plunge, the new (tougher) chip-resistant ST-C will be available. I've been advised by a dealer in Dubbo, who provides a lot of tyres "out west" to Bourke and beyond, that they recomend (and sell a lot of) BFG M/Ts. They claim are ideal for the type of country out there. Smile, you're on ExplorOz
Rohan (Sydney - on the QLD side of the Harbour Bridge)
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FollowupID: 22835

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