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)