4WD Tyre Size

Submitted: Tuesday, Feb 26, 2013 at 16:24

ThreadID: 100776 Views:10613 Replies:7 FollowUps:13

This Thread has been Archived

Tyre Size Calculator

Use this page to work out the radius, circumference and diameter of any size tyre. You can also run comparisons between different tyres and work out the differences in size and odometer corrections.

Warb

Prompted by an article in a recent magazine, which stated that +10% radius tyres were legal on a 4WD, I carried out some further investigation. The author of the article (by email to me) states that VSB14 allows +50mm diameter tyres to be fitted to 4WDs, and so they are legal.

On reading VSB14 in depth, things are not quite as simple. Yes, it does state that 4WDs can have +50mm diameter tyres fitted without "engineering inspection", but only if the total increase in height of the vehicle with all modifications is no more than 50mm. So if a 50mm suspension left has been fitted, the tyres can't be changed at all as the 50mm total lift has already been used! This is not mentioned in the magazine article, but is very pertinent as many 4WD's have raised suspension fitted by their owners.

For NSW it gets even more complex. I asked the question directly to the RMS (ex-RTA) and it seems they have not accepted VSB14 as anything more than a guidance document for inspectors working under the VSCCS vehicle certification system. So in NSW, compliance to VSI_09 is still the legal requirement, and it allows only for +/-15mm tyre diameter from the vehicles tyre placard. Any tyre greater than +15mm diameter therefore requires engineering approval in NSW. I do not know the situation for other states/territories.

I realise that this has probably been discussed many times previously, and there are many opinions on the subject. However at this time the NSW RMS have stated to me in writing that +15mm diameter is the legal maximum tyre size, no matter what the magazines might say!!

Back Expand Un-Read 2 Moderator Share

Reply By: mikehzz - Tuesday, Feb 26, 2013 at 16:40

Reply By: Ross M - Tuesday, Feb 26, 2013 at 16:59

Tuesday, Feb 26, 2013 at 16:59

Warb
If the tyre size original is 265/75/16, a 10% bigger radius tyre changes it from a 804 diameter tyre to around 961mm. 10% radius is added above the rim and below the rim to give 20% bigger diameter.

This is a very large amount.
Are you sure the mag mentioned a +10% increase in radius?
Can you clarify what it is?

Plus 50mm diameter of the whole tyre above placard nominated is usually permitted if on a 4wd.

Ross M

AnswerID: 505619

Follow Up By: stan c - Tuesday, Feb 26, 2013 at 17:19

Tuesday, Feb 26, 2013 at 17:19

i called the nsw rta recently about this matter and they told me that you can increase the tyre size by 15mm rolling radius stan

FollowupID: 782553

Follow Up By: Warb - Tuesday, Feb 26, 2013 at 17:40

Tuesday, Feb 26, 2013 at 17:40

In NSW the maximum increase in diameter is 15mm. End of story.

In fact a 10% increase in radius is the same as a 10% increase in diameter:

20 inch diameter +10% = 22inches
10 inch radius +10% = 11inches.... = 22 inches diameter.

And yes, the statement made was "ten percent rolling radius".

FollowupID: 782556

Follow Up By: Ross M - Tuesday, Feb 26, 2013 at 21:05

Tuesday, Feb 26, 2013 at 21:05

Warb
Are you sure the 10% radius thing is only 10% diameter increase?
I think it is 20% and so does my industry tyre size calculator.
To get a 961mm dia tyre is 20% dia more than a 804 tyre.

FollowupID: 782573

Follow Up By: Warb - Tuesday, Feb 26, 2013 at 21:30

Tuesday, Feb 26, 2013 at 21:30

Yes. It's 10%.

You have said that 961mm is 20% more than 804mm, which is entirely correct. But your basic sum was wrong - you added 10% of the diameter, not the radius. Then you multiplied it by 2 to give 20%.

An 804mm diameter tyre is 402mm radius.

Add 10% (40mm) gives a radius of 442mm, which is a diameter of 884mm. That is a 10% increase.

FollowupID: 782576

Follow Up By: Warb - Wednesday, Feb 27, 2013 at 06:03

Wednesday, Feb 27, 2013 at 06:03

By the way, whilst a 10% increase in radius most definitely is the same as a 10% increase in diameter, it has no bearing whatsoever on the legalities of tyre size. In NSW, whatever the original factory fitment, the maximum increase is 15mm in diameter. So if you have a Mini with 10" wheels you can increase the tyre by 15mm diameter (over factory size), and if you have a Landcruiser 200 with 285/65 R17's you are still limited to a 15mm increase. Obviously the percentage increase that 15mm gives on the Mini is far greater than 15mm on the LandCruiser - but the law is 15mm either way.

FollowupID: 782592

Follow Up By: mikehzz - Wednesday, Feb 27, 2013 at 06:50

Wednesday, Feb 27, 2013 at 06:50

Stan, it's 15mm diameter not radius. That means it's 7.5mm radius.

FollowupID: 782594

Reply By: Penchy - Tuesday, Feb 26, 2013 at 17:14

Reply By: Member - Andrew - Tuesday, Feb 26, 2013 at 17:26

Tuesday, Feb 26, 2013 at 17:26

Hi Warb

VSB 14 was a nationally developed document covering the modification of vehicles that was supposed to be picked up as a national standard by all states. Of course this didn’t happen because some states wanted more control over modifications and there were a number of technical issues still being argued. It also seemed to transfer a lot of issues into the hands of engineering signatories to justify that scheme. Version 2 was picked up in Victoria and is now law. The confusion here is that there is another policy document in Victoria called VSI 8 Here that has alternative requirements. What the?
The real issues around VSB 14 Link here seems to be that they are worried about things like electronic stability control (ESC) but can’t tell you what the performance parameters are. The aftermarket industry did a heap of testing to show that bigger wheels and minor lifts were not causing a major problem. (We are not talking the major mods pushed by one magazine as necesary just to go shopping)
The issues around tyre sizes have become very confused because 4x4’s are big things with a chassis that go offroad or down mines. At least that seems to have been the experience of the decision makers. The development of allwheel drives, softroaders etc has confused the thinking because they can’t decide what is what. Just look at the mess around what is an exempt 4 wheel drive when it comes to fitting snow chains.
This is one of those areas where the 4 wheel drive associations need our support to argue for a practical and national approach.
Regards

A

AnswerID: 505624

Follow Up By: Warb - Wednesday, Feb 27, 2013 at 08:22

Wednesday, Feb 27, 2013 at 08:22

Some of the confusion seems to spring from the attempt to apply different regulations to "4WD's". The +50mm tyre size section is a case in point - it applies specifically to 4WDs, and is an exemption from the more stringent regulations that apply to other passenger "cars". The trouble with exemptions is that then everybody wants to use the exemption if it benefits them - as you say, what comprises a "4WD"? Many of the supposedly "real 4WD's" are no longer chassis based, beam axle vehicles. What differentiates a new monocoque and independent suspension Pathfinder from a Subaru Outback?

Then of course the exceptions start to require additional exceptions - as I read VSB14 it seems that 4WD's are allowed (by an exception) a +50mm tyre diameter, but it is nowhere stated that this in any way impacts the overall +50mm height increase clause. So if we fit an OME spring kit that gives a 50mm suspension lift, we cannot fit larger tyres. Unless they add another exception.....

Honestly, much as I resent the "nanny state" concept, I do see why they have problems making the regulations work!

FollowupID: 782599

Follow Up By: blown4by - Saturday, Mar 02, 2013 at 23:17

Saturday, Mar 02, 2013 at 23:17

Warb,to answer your question: "what comprises a "4WD"?", it is a MC category vehicle which under the ADRs is a passsenger vehicle with off-road capabilities. It must have a certain ground clearance (hence 4WDs cannot be legally lowered) and the entry and departure angles are specified. One would normally expect such a vehicle to have low range also. The category is stamped on the compliance plate. It does not include AWDs and other 'pretend' 4WD or 'soft' off-roaders. The reason for limiting the increase in tyre diameter to 50mm is that the larger the diameter the tyre the larger the reduction in braking efficiency. Also the reason for limiting the overall lift of a 4WD to 50mm without engineering approval and a lane change test is that 4WDs by their very nature already have a high centre of gravity so any further increase in lift further increases the CoG. These rules may seem like bureaucratic red tape but they are all based upon sound engineering principles and are for the benefit and safety of all road users and vehicle occupants.

FollowupID: 782832

Follow Up By: Warb - Sunday, Mar 03, 2013 at 08:45

Sunday, Mar 03, 2013 at 08:45

I agree entirely and fully understand the reasons. Unfortunately, as I said, ANY modification or set of modifications can be dangerous even when completely legal. For example, keeping the normal "on-road" tyres fitted to a modern 4WD, whilst raising the suspension with soft springs and shocks and then fitting a tall roofrack with a heavy load results in an increase in COG and decrease in resistance to bodyroll that will significantly impact the handling. Combine that with a lack of understanding of the limitations of such a vehicle (the car-like interior of a modern 4WD, together with talk of chipping and throttle remapping suggests many 4WD drivers refuse to accept the inherent limitations of their chosen vehicle) and we have problems, yet are entirely legal.

The result is that being safe comes down to the modifier/driver of the vehicle, and the application of common sense. Unfortunately common sense is actively discouraged these days, so the rules need to replace it, and that is hard when many people don't know the rules or simply choose to ignore them....

ADR category MC includes "softroaders". An attempt is made to segregate 4WD's and AWD's, but this is fraught because many of the definitions of AWD and 4WD are quite wishy-washy and there are exceptions to almost every example. Locking centre diff; not on a Disco II. No low range on AWD's; there is on a Subaru Outback. My point however was that each generation of 4WD's is more refined, more car like and less "4WD-like", making them more and more difficult to distinguish from anything else.

FollowupID: 782839

Reply By: Hema Maps - Wednesday, Feb 27, 2013 at 15:08

Reply By: member - mazcan - Thursday, Feb 28, 2013 at 11:22

Thursday, Feb 28, 2013 at 11:22

hi warb
thanks for bringing this item to everyones notice
it is very interesting to read the follow-up threads as well and it seems that this is another item at which all states have different aspects of what should and shouldnt be legal in respect to the article
nsw and victoria it appears are different and i havent delved into wa or the other states
and yet the vehicles with various mods can travel in and across all states
so when are the levels of mods going to become standard at a national level of regulations
instead of having a mixed bag of mods that can travel anywhere regardless
we are one big country so why do we have so many different and confusing rules
for not only this but also other things
just my 2cents worth
cheers

AnswerID: 505729

Follow Up By: Warb - Thursday, Feb 28, 2013 at 12:03

Thursday, Feb 28, 2013 at 12:03

I often find myself wondering this. Australia has total population about the same as some large cities in other countries, yet we manage to have 8 (if I've counted correctly) state/territory governments, a federal government plus regional councils. The result is inefficiency and huge cost! Not only that, but the differing rules cause problems on a day to day basis. Road rules change at a river crossing, so what is legal on one side is illegal on the other. The rural press published articles about this a while back, because farmers and contract harvesters had machines that could be driven legally in one state but were "busted" when driving 500m down the road to a paddock in the next state.

In this particular case we (the taxpayers) seem to have have paid for a group of experts to come up with a "national standard", and now we are paying for 8 more groups of experts to remove the commonality and recreate it as 8 differing versions, thus removing the very purpose for which it was designed!

Still, we've got to keep all those "public servant" jobs going, haven't we?!!

To extend your point, if a passenger vehicle travels to another state, does it have to comply with that states legislation? I am told that agricultural vehicles have to comply with the state they are actually in (as mentioned above), but would it be legal to drive a QLD registered vehicle with +50mm tyres (legal in QLD) through NSW where they are illegal?

FollowupID: 782676