Outer Limits 4x4 Board

Ok so i now have a copy of the standards manual (2002) released by "The Tyre and Rim Association of Australia". This is what police and qld transport refer to to determine wether or not your tyres are too big, small, wide etc.

The tyre placard on my HJ60 lists a recomended tyre size of 7.50R16LT

In the Standards Manual the size of a 7.50R16LT tyre are listed to be
804mm Highway Tread(31.65"), 812mm Traction Tread(31.96").

Now according to Qld Transports modification leafelt, http://www.transport.qld.gov.au/qt/driver.nsf/files/images/$file/Vehicle-Modifications.pdf I am allowed to increase the tyre diametre by 15mm, thus for a traction tyre the maximum size allowed for my HJ60 is 827mm (32.56").

i have a set of Kuhmo 33x12.5R15 All Terrains on my car, these measure a diametre of 810mm(31.9") and i have a set of 31x10.5R15 which measure 740mm(29.13").

I find it interesting that both the 33" and 31" tyres are smaller than thier associated sizes, i have heard people say previously that tyres sizes vary a great deal, but i never would have thaught a full inch smaller (for the 33's) and nearly 2 inches smaller (for the 31's).

so basically, my HJ60 is legal with some 33's and not others. the BFG T/A for instance are specified to be an overall diametre of 32.7, whilst the Kuhmo AT's are specified to be 32.3.

just thaught some people might be interested on the absolute maximum size for a 60 series.

If you give me what your tyre placard says i can give you what the Standards Manual specifies to be the overal diametre if your interested.

for instance,

My old mans hilux specifies 255/70/r15 which according to the manual is 739mm(29.1"), so the maximum size tyre he is allowed is 754mm(29.7")

most hilux's come with 205R16LT which has a highway tread size of 736mm and a traction tread size of 747mm

generally the transport guys wil allow 33" tyres on most vehicles if you run 35" then i spose you could claim the "actual" tyre size is smaller

Quick questions....

Are you measuring these on the vehicle?
How much tread is left on them?

The fact that tyres have an average of say 12mm of tread new, this can give a varience of 21mm overall Diameter from new to just legal.
The loaded radius of a tyre is less aswell.
I believe that tyre manufacturers are allowed up to a 2% varience which gives them a +2% or -2% varience on sizing.... and still be able to class it as that particular size.

There has been great debate in the past between the DOT about the tyre size increase on 4wd (as the 15mm was more for passenger vehicles). A mate of mine who is also in the tyre industry has been told by a couple of inspectors that as long as the vehicle looks to be in good repair and the tyres aren't blatantly oversized for the vehicle, they don't have a problem with larger tyres.
It's still a very gray area.

yes measuring the 33's on the vehicle, 31's off

was measuring sideways on the 33's, and they are about 50% or more id say.

i also thaught the increase was a full inch for 4wd as opposed to the 15mm, b ut the modifications leaflet would appear to state otherwise.

i'd be very interested in the standard sizes for a 10R15...

I found it strange that neither the RTA or 3 major insurance companies could answer this question for me...

made me think that noone really cares.

Syzygy wrote:i'd be very interested in the standard sizes for a 10R15...

I found it strange that neither the RTA or 3 major insurance companies could answer this question for me...

made me think that noone really cares.

10R15LT is classed as a Wide Based Tyre, Highway Tread 773, Traction Tread 781, Says they need an 8" rim aswell. Also mentions them to be mounted on 5 degree drop centre rims, whatever that is.