Monday, Feb 22, 2016 at 16:15
There are a lot of misunderstandings about tyre sizes and what you can fit. It is because most people do not read past the headlines.
All states adhere to the national modification rules. If you comply with these rules, your vehicle will be legal in every state.
The national modification rules are known as VSB 14. The "LS" rules within this regulation deal with tyre sizes.
Everyone jumps to section 4.2 in these rules. This is the one that lists the 50mm increase, however, this rule applies when you are changing rims and tyres.
Section 4.1 deals with what tyre sizes you can fit if you keep the standard size rims. It states:
4.1 REPLACEMENT TYRES ON STANDARD (OR MANUFACTURER’S OPTIONAL) RIMS
Some tyre sizes have become obsolete and are no longer available. Equivalent replacement tyres with different size designations may be fitted without specific certification under this Section.
Every passenger car manufactured after 1972 (ADR 24) is fitted with a tyre placard that contains information on original and optional tyres and rims for that vehicle model.
A motor vehicle under 4.5 tonnes GVM which is required to comply with ADR 24 may be equipped with tyres other than those listed on the tyre placard provided that:
• • the load rating of the tyres is not less than the lowest load rating listed on the tyre placard of the vehicle or equivalent variant of that model vehicle;
• • the speed rating of the tyres fitted to a passenger vehicle is at least 180 km/h (S) when the tyre placard requires a higher speed rating than S;
• • the speed rating of the tyres fitted to vehicles with special features for off-road use of at least 140 km/h N when the tyre placard requires a higher speed rating than N; and
• • for all other vehicles a speed rating of at least 120 km/h.
• • In special circumstances, the speed rating may be less than the ratings specified above if the speed rating of the tyre is more than the vehicle’s maximum speed. This will need to be confirmed with the Registration Authority
I read this as you can fit any diameter tyre to the standard rims as long as they can be legally fitted to those rims. That is, tyres will have a rim size listed to what they can be fitted to. There is certainly no 50mm limit listed in these rules.
Now we can move on to section 4.2 if you are changing your rims. The biggest mistake people make is thinking that there is a specific diameter listed somewhere for each tyre size. This is not the case. You only have to look up the actual diameter of various tyres. Mud tyres tend to have a larger diameter for a specific size than say a road tyre, the diameters also vary between manufacturers. So what is the base diameter than I can go 50mm above.
ADR 23 deals with the requirements of tyres and their sizes. Up to 2009, there was only ever a minimum diameter specified for a given tyre size. So basically a given tyre size could have any diameter above a certain measurement. So if there is not requirement under the ADRs to have a certain diameter for a particular tyre size and the modification rules allow you to go 50mm above the diameter of the tyre size, it is very open ended about how large a tyre you can fit.
In 2009, ADR 23 actually brought in a range of diameters for a given tyre size. They came into line with the
United Nations Regulations. These rules allow a minimum and maximum diameter for the size in question. Basically you calculate the theoretical diameter the same way as most of you do, that is, using the rim size, tyre width, and profile. The regulations state that for a 4WD tyre, the range in size can vary from 3% below the calculated diameter to 6% above.
So with a range of diameters that can easily range by 20 to 30mm, which diameter do you use in calculating how big you can go. I would suggest that you can use the maximum diameter as this is the diameter most closely related to mud tyres and if we are putting on bigger tyres, they are usually mud tyres.
But you can interpret these rules how you like. I am just highlighting that there is a little more behind the headline rules that everyone keeps on quoting
AnswerID:
596505
Follow Up By: Robin Miller - Monday, Feb 22, 2016 at 16:35
Monday, Feb 22, 2016 at 16:35
Hi Matthew
Thats good info and one reason why you do see, and hence can optimize a given tyre size for your application.
Its not however the issue in this case , in this case the same product is given different descriptions in two different publications from the same company, subject to the same standards.
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865376
Follow Up By: imatthew - Monday, Feb 22, 2016 at 16:44
Monday, Feb 22, 2016 at 16:44
Robin,
It is a matter whether the tyres are within the "standards". I would think that what you got was within the standards for that type of tyre of that size.
Then your only point for redress is whether they falsely advertised the tyres. If you got the diameter off a current official document from the company and they needed to be that diameter to fit the purpose you required them for, then they have to give you a refund under Australian consumer law. That is, the tyres were not fit for purpose. But the wrong diameter would have to be on a current official document provided by the person you purchased them from. If it is just off something that you found on the internet, I would not like your chances.
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Follow Up By: Robin Miller - Tuesday, Feb 23, 2016 at 08:03
Tuesday, Feb 23, 2016 at 08:03
No worries Matthew , fortunately the wrong figure is actually in the companies brochures.
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865400